Of the Primary Division, or Province, of the Animal Kingdom, which is the subject of the present Article, Aristotle first defined certain members, under the name "Malakia," of which "Mollusca" is a rude Latin equivalent.

The great and peculiar merit of the system of Cuvier, as exemplified in the Règne Animal, and especially in the last edition (1830) which came from the hands of the illustrious author, is the determination of the characters and boundaries of the molluscous province by investigations of the entire animals.

Most Mollusks are protected by a shell, and the shells had monopolized the attention of most preceding classifiers. After Cuvier, "Conchology" sank to its proper position as an artificial system and a subordinate department of "Malacology." By Malacology is meant the science of the "Malakia," Mollusca, or Mollusks; as "Ichthyology" is that of the Pisces, or Fishes. We now, in English speak of "a mollusk" and of "mollusks," as the French of "mollusque" and "mollusques." Subsequent advances in the knowledge of Mollusca have been made chiefly by pursuing the anatomical methods of Cuvier, and by applying them not only to the last or fully developed stage of the individual, but to the previous stages of its development, from the egg onwards. These embryological investigations have required the use of the microscope; and Malacology, like other branches of natural history, has benefited by the application of the higher powers of that instrument to the investigation of both the minute embryos and of the component tissues of the organs of the Mollusks.

The first important improvement in the characters and constitution of the province or sub-kingdom of Mollusca, as defined by Cuvier in 1794 and 1817, was the elimination therefrom of the order Cirripedia. In 1819 Straus Durckheim, in a memoir on the Structure and Affinities of an Entomotarous Crustacean (Daphnia), published in the fifth volume of the Mémoires du Muséum, compared in detail the organization of a Cirriped (Pentelasmis) with that of an Entomotarous (Limnadia), and pointed out their close affinity; which affinity was, two years after, insisted upon and illustrated by the correspondences of structure between Pentelasmis and Daphnia, by Mr W. S. Macleay, in his Horae Entomologicae, part i., p. 308. Dr Gray accordingly omitted the Cirripedia from the "Systematic Arrangement of the Mollusca," which he published in the London Medical Repository, vol. xv., 1821, p. 229. But the capital discovery, proving the plan of structure and the essential nature of the Cirripedia to be "articulate," not "molluscous," was made by Mr J. V. Thompson, and was published in 1830 in the third part of his Zoological Researches. His observations were made on the larva of a small sessile barnacle (Balanus pusillus), which he showed to undergo a very remarkable metamorphosis. He captured their larvae in a fine towing net, as they swam freely in the guise of a small Crustacean furnished with a flexible shell composed of two valves like those of Daphnia. Being preserved alive in a vessel of sea-water, these larvae cast off their bivalve shell, attached themselves to the bottom of the vessel, and became transformed into the Balanus pusillus of Pennant. Whilst the young animal possesses the natatory limbs and the power of locomotion, it has a pair of compound eyes, which gradually become obliterated, as in the case of the crustaceous Cymothoë, in the final transmutation to a fixed and sessile state. Other phenomena detailed by Mr Thompson left no doubt that the homogangliate type of the nervous system of the Cirripedia, first made known by Cuvier, most truly indicated the primary group or province of the animal kingdom to which that singular class of shell-clad animals belonged.

Burmeister, who corroborated the discovery of Thompson, formally proposed, in his work on the Cirripeds, published in 1834, to transfer them to the Articulate province, and to place them as a particular tribe in the class Crustacea.

De Blainville had indeed included the Cirripeds with the Chitons in a distinct sub-class called "Malentozoaires," under which name they are described, as the order Nematopodes, in the work on Mollusca called Malacologie, which was published as such in 1825. But the obvious want of anatomical knowledge, or indifference to evidence from organization, which the association of the Cirripeds with the multivalve Gastropods displayed, prevents the attachment of any importance to the innovating ideas and sweeping changes of malacological nomenclature proposed by this naturalist.

For the same reason, no influence on the modification of molluscous classification was exercised by the suppression of Cuvier's order Pteropoda, and the intercalation of the genera of that order between Helix and Fissurella amongst the Gastropods of Cuvier, in the Manuel de la Malacologie of 1825 above cited. Even as late as 1837 Professor Milne Edwards, in his Éléments de Zoologie, adopts the constitution and relative position of the Pteropoda assigned to them by Cuvier in the last edition of the Règne Animal; and their position is preserved unchanged in the second volume of the Zoologie de la Bonite, published by M. Souléyet in 1832.

But before noticing the chief modifications of the Cuvierian system in reference to minor groups within the limits of the Molluscous province, it may be remarked, with reference to those limits, that had Cuvier placed due reliance on the character of the nervous system which had previously guided him in his primary arrangement of the animal kingdom, he might have been led by his discovery, in 1815, of the homogangliate type of that system, in both sessile and pedunculate barnacles, to the reform subsequently worked out by Straus Durckheim and others.

The modification of two of Cuvier's provinces by the subtraction of the class Cirripedia from one, and the addition of these soft inarticulate conchiferous animals to the other, induced Professor Owen, in the Synopsis of his course of Lectures delivered at St Bartholomew's Hospital, and published in 1835, to propose the term Heterogangliata for the so-curtailed Mollusca, and Homogangliata for the pro-

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1 See Mémoires pour servir à l'Histoire et à l'Anatomie des Mollusques, &c., 1817; a volume which consists of a series of papers published chiefly in the Annales and Mémoires du Muséum d'Histoire Naturelle, from the year 1792 to the date of their collection into that volume.

2 "Second mémoire sur l'organisation et les rapports des animaux à sang blanc, dans lequel on traite de la structure des Mollusques et de leur division en ordres" (Magazine Encyclopédique, t. li., an. iii., 1794).

3 Le Règne Animal distribué d'après son organisation, 8vo, 1817.

4 "Mémoire sur les Animaux des Anatifes et des Balanes, et sur leur Anatomie" (Mémoires du Muséum, tom. ii., 1815). portionally expanded Articulata,—terms expressive of the leading modifications of the nervous system characterizing those provinces respectively. The term Mollusca is used in the present article, as by most recent authors, in the sense of the Heteroangliata as above defined.

The Mollusca are here meant to include the animals with one ganglion or more below the entry to the alimentary canal, whence nervous chords radiate to form a collar round that entry, and to supply other organs of the body; nervous ganglions being superadded, as the species ascend in complexity of structure, above the gullet, representing a brain, and in different parts of the body, but commonly in a more or less scattered or unsymmetrical manner, and never along a pair of symmetrical sub-abdominal median chords.

In the highest molluscous class (Cephalopods) muscles may originate from an internal cartilage; in other Mollusks they are attached to the skin or to the shell which may be developed in that otherwise soft substance. The shell is hardened chiefly by carbonate of lime, and consists either of one piece or two pieces, called "valves;" rarely of more than two, or with accessory parts. The blood is colourless, not red; the heart is distinct, muscular, and propels the blood through a system of arteries and veins, the latter having more or less the form of irregular sinuses or lacune. The respiratory cavity, with the exception of one family (Ascidians, which therefore ought rather to be transferred to the molluscoid Zoophytes), receives or opens near the anus. The intestine is usually bent or reflected forward to effect that relation. The Mollusca may be either dioecious or hermaphrodite. The molluscoid Zoophytes are parthenogenetic.

The foregoing characters of the Molluscous province are natural, if the Ascidians and ascidifoid Polypes are excluded. Admitting these as the lowest confines of the province, its definition must be qualified by large exceptions.

Cuvier, in publishing the collected results of his anatomical investigations of the Mollusca, remarks in the preface:

"Here may be seen to what point some among them approach the vertebrate animals, and one may convince himself that it is no longer possible to leave them, as Linnæus did, in the lowest class of the animal kingdom confounded with the tentaculiferous Polypes and other as simple Zoophytes."

Savigny was the first to show that such linking on of the molluscous to the polype type of structure was not so impossible in nature; this he did by his discovery of the organization of the compound Ascidians which had previously been classed with Zoophytes.

Audouin and Milne Edwards, Ehrenberg, Thompson, and Arthur Farre—the last-named author more especially—have demonstrated the resemblances in organization between the compound Polypes with ciliated tentacles and the compound Ascidians.

These results of anatomical investigation, carried out into forms of a lower grade than the Mollusks dissected by Cuvier, together with subsequent embryological observations demonstrating certain developmental characters which

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1 "On y verra à quel point plusieurs d'entre eux se rapprochent des animaux vertébrés, et l'on se convaincra qu'il n'est plus possible de les laisser, comme l'avait fait Linnæus, dans la dernière classe du règne animal, confondus avec les polypes à bras et d'autres zoophytes aussi simples." (Histoire des Mollusques, &c., 4to, 1817, Avertissement, p. v.)

2 "Mémoire sur les Animaux Composés (Mem. de l'Acad. des Sciences, Paris, 1817); Mémoires sur les Animaux sans Vertèbres, 8vo, 1816-18.

3 Rapport fait à l'Académie des Sciences, Paris; and Annales des Sciences Naturelles, 1828.

4 Symbola Physica, 1828-1831 (Bryozoans).

5 On Polypes, a new animal discovered as an inhabitant of some Zoophytes," in Zoological Researches, 1830.

6 Observations on the Minute Structure of Ciliobrachiata Polypes" (Philosophical Transactions, 1847).

7 "Il nous semble que la division de Linnæus est encore la plus naturelle. Elle est en effet le plus en harmonie avec le développement."

8 Chez les vers de Litonge, dont Cuvier a fait les Mollusques et les Radiolaires, le vitellus ne rentre pas le dos ni par le ventre; on pourrait les désigner sous le nom de Allotaxillians ou Allotaxipodons. Les Allotaxillians comprennent la classe des Mollusques, des Polypes, des Echinodermes, &c." (Van Beneden, Embryogenie des Bryozoaires, 4to, 1845, p. 7.) bent or spiral arms, compared by Siebold to those of *Alegonella*, conduct to certain forms of *Bryozoa*, to which they are more closely allied than is the *Salpa* or *Ascidia*. The progress to the latter form of Mollusk may be traced upwards as follows:

Amongst the varied forms of microscopic beings called *Infusoria* and *Polygastria* is one (fig. 3, A) shaped like a bell, attached by a contractile apical peduncle to a foreign body, with the base of the body provided with vibratile cilia, effecting the respiratory currents of the surrounding water, and drawing the food towards the mouth, which, with the vent, open upon the same ciliated extremity of the body. The ciliated border is retractile. This organism propagates, among other modes, by buds (fig. 3, B), which become detached, and for a time swim freely as "ciliated gemmules" (fig. 4, C).

In another minute aquatic organism, with a similar form and pedunculate attachment (fig. 4), there is a certain advance in development, with concomitant distinction of parts. The vibratile cilia are supported on tentacular prolongations g of the free base of the bell-shaped body A. The cavity cd receiving and acting on the food, is now distinct from the proper walls of the body, and can be traced, as a canal, extending from the mouth e to the vent d, both of which, however, open, as in the *Vorticella*, within the ciliated circle. The contractile fibres m, which retract the ciliated borders, are manifest.

The organisms showing this advance of structure are called *Bryozoa* and "cilibrachiate Polypes." They propagate by buds B, and by free-swimming ciliated gemmules C. The generic form here illustrated is called *Pedicellina* (fig. 4).

A third type, or what, in regard to general development, might be called a third stage (fig. 5), is manifested by a group of small aquatic animals called Ascidians, for the most part compound, and in some species pedunculate, as in the two preceding stages; the peduncle being attached to the apex or end of the body opposite to that on which the mouth and vent open. But in the present more advanced type the walls of the body are so developed as to surround separately the oral (q) and anal (f) apertures; rudiments of ciliated tentacles g are confined to the oral inlet, and the cilia, which answer in function to those of *Pedicellina* and *Vorticella*, are developed from a capacious sac cc, with transverse rows of holes, at the lower part of which the alimentary canal begins; this canal d returns upon itself, as in the former more simple types, and terminates at k, below the anal aperture of the body f. A rhythmically contracting or pulsating body exists in all the three grades of structure above cited. In the Ascidian type the heart n manifestly circulates a nutritive fluid through two channels, marked by arrows in fig. 5, which convey it in opposite directions along the stem, which thus organically unites the different individuals supported by its branches.

The Ascidians propagate by buds (B) and by eggs, the latter excluding free-swimming larva C, shaped like the tadpole of the frog. This well-marked developmental distinction has not received from some naturalists the importance it merits in the question of associating the *Bryozoa* with the *Tunicata*.

As to their relations to the true *Mollusca*, some classifiers draw the line between the first and second steps of development; others between the second and third. One physiologist assigns developmental or embryonic facts in support of an incorporation of all *Zoophytes* with the *Mollusca*; another physiologist would associate the rhizobranchiate and cilibrachiate *Acalephae* with the Salpae and other acephalous *Mollusca*. The philosopher who may, in the range of his studies of animated nature, include the highest phenomena of life—the psychical—will be apt to believe that the systematist who is really least qualified will denounce most dogmatically every partition which does not tally with his own notions of the proper boundary line.

It is here proposed to submit that the *Mollusca*, for the

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1 Van Beneden, loc. cit. 2 De Blainville, *Considérations sur les Animaux et leur Classification*, Svo, 1840, p. 31. ends of definition and description, should be understood to commence, in the ascending order, with those forms in which the respiratory organs or cavity are in relation rather with the vent than with the mouth. The oral relation with the ciliated or tentaculiferous circle prevails from the *Vorticella* and *Hydra*, to the *Ascidia* and *Terebratula* inclusive; but the bend of the intestine in *Discina* and *Lin- gula* indicates the tendency to the usual molluscous relation of the vent in the *Brachiopoda*.

In deference, however, to the expectation of most readers, and with the conviction, when the progressive series of developments to different modifications of the molluscous type have been traced and illustrated, of the essential unimportance and artificiality of any line defining the lower boundaries of the heteroganglionic province, the present article will include, with the anal-breathing classes, the *Tunicata* and *Brachiopoda*, although these are rather "Molluscoid" than *Mollusca*.

Having premised so much as to the definition and extent of the molluscous or heteroganglionic province, the subject of its partitions, and of their relative value in the scale of organization, is next in order.

In tracing the progressive complication of animal structures from the simplest forms, we soon find ourselves travelling along two distinct routes, the additional complexities adjusting themselves according to two general types of organization, which are, however, so gradually assumed, that the lower boundary must in both cases be arbitrarily defined.

In the great province of the *Articulata* the advance of structure is most conspicuous in the organs peculiar to animal life, and is manifested in the powers of locomotion, and in the instincts which are so various and wonderful in the insect class.

In the *Mollusca* the developmental energies seem to have been expended chiefly in the perfection of the vegetal series of organs, or those concerned in the immediate preservation of the individual and the species.

The *Mollusca* are so called on account of the soft unjointed nature of their external integument. The scattered centres of the nervous system, disposed according to the heteroganglionic type of that dominant system of organs, is often accompanied with an unsymmetrical form of the entire body, which, in compensation for the low condition of the perceptive energies, is protected in most of the species by one or more dense calcareous plates, called shells.

All true Mollusca have a complete alimentary canal, with mouth, stomach, intestine, and vent; and they are provided with circulating and respiratory organs.

In a large proportion of the lower organized Mollusca there is no head and no brain or superoesophageal ganglion. The inlet for the food is simply a pharynx, or beginning of the oesophagus, without jaws, tongue, or mouth, properly so called. All other *Mollusca* are provided with a head, which generally supports feelers, or soft tentacula, eyes, and a mouth armed with parts for mechanically operating on the food.

The molluscous province may thus be primarily divided into *Acephala* and *Encepha- lous*.

The Acephalous Mollusca are all aquatic, and are divided into classes according to the modifications of their integument, or of their gills.

The *Tunicata* are those which are inclosed by an elastic cellulose uncalcified integument. They breathe either by a vascular ciliated pharyngeal sac, or by a riband-shaped gill stretched across the common visceral cavity. John Hunter, who had anatomized the typical forms of this class, and had recognized the homology of their flexible intro- case to the shells of the bivalves, to which Mollusks he saw that Banks's "Daggyza," and the "squirters" of our own shores, were most nearly allied, grouped together the *Salpe* and *Ascidia*, as they are now called, into a natural family, which he termed "Soft-shelled." This family is the same as that afterwards described and called "shell-less Acephala" by Cuvier, and *Tunicata* by Lamarck. All the other Ace- phala have a bivalve shell.

The *Brachiopoda* have two long spiral arms developed from the sides of the mouth, and respire chiefly by means of their vascular integument or "mantle." One valve of the shell is applied to the back, and the other to the belly, of the animal, which is attached by its shell, or by a pedicle, to some foreign body.

The *Lamellibranchiata* are bivalve conchiferous Mollusks, which respire by gills in the form of vascular plates of membrane attached to the inner surface of the mantle. One valve is applied to the right side, the other to the left side, of the animal. The common oyster and mussel are examples of this best-known class of Acephalous Mollusks.

The Encephalous Mollusca are divided into classes according to the modifications of the locomotive organs.

The *Pteropoda* swim by two wing-like muscular expansions, extended outwards from the sides of the fore part of the body.

The *Gasteropoda* creep by means of a muscular disc attached to a greater or less extent of the under part of the body.

The *Cephalopoda* have all or part of their locomotive organs attached to the head, whence they radiate in the form of muscular arms or tentacula.

In the last class only do we find, in the present series of animals, an internal skeleton, combined in some with a shell. In the rest of the Mollusca the hard parts, where present, are external. But the integument in certain species of the Encephalous classes, and in most of the Cephalopoda, is uncalcified and flexible.

The chief modification of the Cuvierian system, in regard to the sequence or relative position of the classes of the *Mollusca*, has been made in the Encephalous division, and consists in the transference of the *Pteropoda* from above to below the *Gasteropoda*.

The grounds for this change, which, as already shown, was not accepted up to 1852, rest mainly upon a demonstrated nearer affinity of structure of the Cephalopods to the Gastropods, and on the resemblance of the mature form of the Pteropods to the larval stage of certain Gastropods.

The first step in the work requisite to establish the necessity of this innovation upon the Cuvierian system will be found in the following passage from Owen's *Memoir on the Pearly Nautilus*:

"The interesting character which the *Nautilus* sustains as an osculant form between *Cephalopoda* and *Gasteropoda*, will perhaps be considered to have been sufficiently manifested by the instances of affinities already cited; but it is rendered still more evident on a consideration of the nervous system. The forms, proportions, and disposition of the principal masses of this system appeared, indeed, at first sight to recede so far from the type of the higher Cephalopods as to have rendered it necessary, in the description, to refer to a Gasteropodous genus in illustration of it. The essential difference, however, as has already been shown, consists in the simpler condition of the central mass; the source of volition being thus in harmony with the diminished energies of the muscular, and the contracted sphere of the sensitive system.

"The differences in the distribution of the principal..." nerves are not less important in a physiological point of view. In the Cephalopods, whose shells are rudimentary and internal, and whose bodies are enveloped in a naked, and, as we must suppose, sensible mantle, the nerves which supply that part radiate from a ganglion, which, as in the posterior roots of the spinal nerves in the Vertebrata, is interposed on the cord which brings them in communication with the central mass. In Nautilus, on the contrary, whose body is incased in an insensible calcareous covering, the analogous nerves are wholly expended on the largely-developed muscles which attach the shell to the body; and these nerves, like the motor filaments of the spinal nerves, pass into the muscles directly from the brain without the interposition of any such ganglion.

"The inferiority of the more intellectual senses, sight and hearing, is in correspondence with the simplicity of the brain; and in receding from the higher Cephalopods in the structure of the eye, it inclines more directly towards the Gastropods, numerous genera of which, and especially the Pectinibranchiata of Cuvier, present examples of this organ analogous in simplicity of structure, and in a pedicellate mode of support and attachment to the head. As the pearly nautilus, like the latter group of Mollusks, is also attached to a heavy shell, and participates with them in the deprivation of the locomotive instruments of the Cephalopods, we may thence deduce the more immediate principle of their reciprocal inferiority with respect to the visual organ; for what would it avail an animal to discern distant objects which could neither overtake them if necessary for food, nor avoid them if inimical to its existence?

"As the spheres of vision and of action, however, are thus limited, the power of taking cognisance of proximate objects is proportionably augmented, and the organs of the simpler sense of touch are more amply developed. In the numerous and singularly-disposed tentacles of Nautilus, we have also examples of a recurrence of structures hitherto unknown among the Cephalopods, and whose analogues are to be sought for in inferior groups. And here again, as in the case of the eye, after searching in vain among the Pteropodous genera, we are compelled to admit the claims of the Gastropods to a closer alliance with the highly organized class whose affinities the pearly nautilus has tended so materially to elucidate. Thus Doris, Tethys, and Tritonia each present examples of sheathed and retractile tentacles; and in the former of these genera they have the same peculiar structure as is displayed in the ophthalmic tentacles of Nautilus." (See Cuvier, Mem. sur le Doris, p. 12, pl. 2, fig. 1.)

"On a consideration of the generative system, it will appear that, as far as regards the female, the pearly nautilus does not recede materially from the Cephalopod type; and it may be remarked that the Pectinibranchiata, in their dioecious mode of generation, approximate closer to Nautilus than the other Gasteropoda, and present a similar laminated glandular organ in the branchial cavity, whose office is supposed to be to secrete the receptacles of the ova after they are expelled.

"The retractile tentacles of Clio are constructed on a plan very different from those of Nautilus. The consideration, indeed, on which the Pteropoda have been placed in the Regne Animal next in order after the Cephalopoda, and preceding the Gasteropoda, appear to be slighter than have usually influenced the immortal author of that work in the position of his groups. Their swimming like the former animals is a relation of analogy; whilst their inferiority to some, at least, of the Gasteropodous families is evidenced by the doubtful nature of their organ of vision, and by their hermaphroditical mode of generation."

Influenced by these views, the author of the work above cited placed the Pteropoda below the Gasteropoda in the Synopsis of his Lecture on Comparative Anatomy, published in 1835. The Pteropoda have the same position in the two editions (8vo, 1843 and 1855) of his Lectures on Invertebrate Animals.

Mr Woodward has adopted the same sequence of the classes of cephalous Mollusca in his Excellent, comprehensive, and accurate Manual of the Mollusca, 8vo, 1851-1856; and it has received the sanction of Professor Van der Hooven in his Handbook of Zoology, second edition, 1855. Thus the position originally assigned by Lamarck to the Pteropoda below the Gasteropoda, by which they are separated from the Cephalopoda, is that which appears now to be generally adopted; and it is perhaps to the genera Bullaia and Gastropterion, and to the Firolidae, that the different forms of this interesting group of floating Mollusks present the closest affinity.

With regard to the subdivisions of the classes, Cuvier has left little of importance to be proposed in the way of amendment; but some innovations have obtained partial acceptance which seem to be retrograde steps.

In the Lamellibranchiate class (the "Acephales Testaces" of the Regne Animal) the extent to which the mantle-lobes are united and developed, the number and position of the adductor muscles, the presence and form of the foot, are still the characters which are chiefly relied on for distinguishing the primary groups of the class.

In the Gasteropoda, Cuvier believed that the modifications of the respiratory organs, especially as to position and form, best indicated the orders of the class. The absence, however, of an articulated skeleton—internal or external—permits the softness of the body of the Mollusk to adapt itself to a greater diversity of undefined and unsymmetrical shapes than is possible in the Articulata or Vertebrata; and hence the relative position of certain organs become changed in a degree beyond anything observable in the primary divisions of animals of more fixed forms. The branchiae, which, in a vermisform Gastropod like the Doris, may project backwards behind the heart, will be found, where the hinder part of the body has to be lodged in the spiral whorls of a shell, to be tilted forwards as by an act of rotation, so as to project forwards and lie in advance of the heart; as, e.g., in the whelk. It has been accordingly proposed to make this really unessential character, founded on relative position and direction of the gills to the heart, over-ride the more important differences in the extent and relative position of the gills to other parts of the body, and also the structure of the gills themselves; and the order Prosobranchiata, in the system of Professor M. Edwards, includes all those Gastropods in which the gills lie in advance of the heart, and thus associates the whelk in the same order with the limpet, and with such floating forms of Gastropod as the Firola and Carinaria; all other gill-bearing Gastropods being "opisthobranchiate." But in reference to the latter term, the student who may anatomize the species which it is proposed to include, will find it to be in many instances an arbitrary, if not an incorrect one. In Glaucon, for example, the gills are external, supported on pairs of processes progressively diminishing in size from before backwards. The heart is situated opposite the middle pair, consequently behind the largest and chief pair; yet the Glaucon is classed with the Gastropods, which have the gills behind the heart. In Diphylidia the heart is near the middle of the body; the gills, commencing near the mouth, extend along each side of the body. In all the Nudibranchiates with branchial tufts some of the pairs are always in front of the heart; and in general, as far as this inde-

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1 Histoire Naturelle des Animaux sans Vertébres, vol. vi., 1818. The absence of any adequate reason for the change in the position of the Pteropoda proposed in this work caused the views of Cuvier to maintain their sway until a comparatively recent period. finite character is concerned, the Nudibranchiata are as entitled to be called "Prosobranchiates" as "Opisthobranchiates."

Some Pteropods are "prosobranchiate;" many might be termed "opisthobranchiate;" but species of the same genus—Cleodora, e.g.—would perplex the zoologist confining in the ordinal character in question by their differences in respect to it. In Hyalea as much of the gill is in front as behind the heart; and one pulmonary vein passes backwards, the other forwards, to the auricle. The really natural character of the group manifesting these varieties in the relative position of heart and gill, shows the soundness of the judgment of Cuvier in founding his orders of Cephalous Mollusca on the more truly important characters of the breathing organ itself. The order Prosobranchiates, in the class Mollusca, resembles in its vague generality the order of the class Mammalia, proposed to be founded upon the discoid placenta.

In regard to a later attempt to generalize molluscous characters according to the course of the alimentary canal, there is less to be said. No care has been taken to distinguish the bend of the intestine which follows and is determined by the part of the body containing it, from the bend of the tube which is independent of any bend or production of the body of the Mollusk. And when the anatomical facts have been too plainly adverse to the proposed classification to be overlooked, they are attempted to be explained away. In the Pecten, the Unio, the Lutraria, and other bivalves, the principal loop of the intestine is open towards the dorsal aspect of the body, or that next the hinge of the shell. But such a bend contradicts the proposed classification on intestinal characters; the author of which is therefore compelled to qualify that principal bend of the intestine as being "accidental." The so-called "neural" Mollusks associate the Polyclinum, Terebratula, oyster, and Clio with the Sepia; the "haemal" Mollusks link the Ascidians with the snails and whelks. One consequence of this classification and nomenclature is to throw doubt on, and to confuse the meaning of, the terms "neural" and "haemal," which are at once definite and intelligible in regard to the Vertebrata, to elucidate the nature of which they were devised, and to which end they ought to be, and doubtless will be, restricted.

The hitherto published illustrations set forth as those of the common plan of structure of the Mollusca appear to be premature; there is at least no evidence of their being the result of researches equivalent in number and kind to those which have issued in the most generally accepted views of the archetype of the Vertebrata.

The ways that have led to the present state of knowledge respecting the vertebrate archetype have been manifold and painfully pursued. The inquiry commenced with a comparison of the connections, during the phases of development, of particular parts in the different vertebrate animals, with a view to determine how far the parts in one species answered to the parts in another in the sense implied by the term "homology."

The grounds for the conclusions as to these "special" homologies have been supplied in detail where those conclusions are enunciated. Upon these conclusions have next been founded a higher series of generalizations, under the term "general homologies;" from which have, finally, been deduced the idea of the common plan of structure of the vertebrate province.

Where special homologies have been arrived at in the attempt to define the molluscous archetype, the results are expressed, according to rule, by the proposition of definite names for the parts so deemed to be determined. Thus a part of the visceral cavity of a Mollusk is called an "abdomen," or a "post-abdomen," as it is placed before or behind the anus; and this definition is an essential basis of the proposed archetype. A general reference to the results of the investigators of molluscous development is made as a warrant for the safety of the above proposition; but the particular proofs are not given; and the writer of the present article has failed to find in the authors cited, or in nature, any evidence to warrant the expression that "the visceral mass is thrust out behind the anus" in any Mollusk.

The growth of the abdomen and intestinal canal, resulting in different relative positions of the anus, is essentially the same in fishes and Mollusks. The different relative positions of germ-yolk and embryo being admitted, the progress of development which places the vent in advance of the visceral mass in the fish, is as little due to a "thrusting out" of such mass as it is in the slug, whelk, or snail. The grounds for pronouncing the part of the abdomen in advance of the anus in a gymnottus, to be the homologue of the part of the abdomen in advance of the anus in an eel, and the part of the abdomen—very considerable in the gymnottus—behind the anus to be a distinct part, are not appreciable by the present writer; he can as little understand the reasons for calling this considerable part of the abdomen a "post-abdomen," and the answerable part according to the contained viscera and the protective portion of the spine in another fish an "abdomen." The mere varying bend of the gut, and consequent different positions of the vent, are wholly inadequate for such a distinction. There is nothing in the development of a Gastropod that makes the position of the anus more adequate for a distinction between abdomen and post-abdomen than in the fishes above cited. The really important modifications in the development of a pulmonated Gastropod, as compared with a Pectinibranchiate, do not effect an equivalent change in the course of the intestine. In the present article it is held that the large cavity containing stomach, intestine, liver, and generative glands in the Doris, is the homologous part with the cavity containing the same viscera in the snail; just as the abdomen in a gymnottus answers to that in an eel, although the vent is at the fore part in the one, and at the hind part of the cavity in the other. The arbitrary division of the great visceral cavity of a Mollusk into "abdomen" and "post-abdomen," inasmuch as it is called for to support the idea of the molluscous archetype here discussed, augurs as ill for the stability of that idea as the like arbitrary assumptions by Spix and Geoffroy did for the stability of their ideas of the vertebrate archetype.

In the paper in the Philosophical Transactions, above quoted, treating of the archetype of the molluscous province, a few other special homologies are enunciated, as, for example, that certain of the cephalic arms—the four next the dorsal aspect—of the cuttle-fish are homologous with the fore-part of the foot in the snail or whelk, the remaining four to the middle part, the superadded tentacles (which arise internal to, and are distinct from, the ordinary arms) to the hind part, the respiratory tube or funnel to the free thickened border of the mantle in the whelk, and to the

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1 See Professor Huxley's paper in the Philosophical Transactions of 1853, p. 45, and in the English Encyclopaedia, art. Mollusca. The same may be said of the term "post-abdomen," borrowed from "Crustaceology," where it has a definite meaning, but not answering to that in which it is applied to Mollusca.

2 Owen, Lectures on the Invertebrate Animals, 8vo, 1843, pp. 374, 379.

3 Owen, On the Archetype of the Vertebrate Skeleton, 8vo, 1848, p. 7.

4 Huxley, Proceedings of the Royal Society, May 27, 1852; Philosophical Transactions, 1853, p. 45.

5 Ibid., p. 46. wing-shaped fins, or locomotive muscular expansions in the Clio and other Pteropoda. The grounds for these special homologies are not detailed in the paper cited: the conclusions themselves are indicated in Phil. Trans., 1853, pl. v., by the symbols pp, ms, mt, and ep, to the four parts, as above successively enumerated.

The difficulty, not to say impossibility, of assigning adequate proofs for such homological propositions depends upon the principle, that the general substance of a Mollusk, and the absence of internal firm parts, influence or relate to the development of muscular tissue according to a teleological rather than a homological principle,—as when a part for adhesion, a part for creeping, a part for swimming, a part for seizing, a tube for expulsion of fluid, &c., is developed, where wanted, agreeably with the exigencies and the sphere of life of the species. This principle plainly guides such developments, viewing the molluscous province generally, more than that law of adhesion to a common pattern which enables the anatomist to view the wing of the bird, the fin of the whale, the fore-paw of the lion, and the hand of man, as answerable parts,—modifications of the same "diverging appendage."

Where grounds for determining homologies, like those which support such generalizations in the vertebrate province, are wanting in the molluscous province, it is better to abstain from speaking of parts as "homologous" until sufficient evidence of homology can be assigned.

To call the four dorsal arms of a dibranchiate Cephalopod "propodium," the four ventral ones "mesopodium," the two tentacles "metapodium," the funnel "epipodium," adds nothing to the true knowledge of cephalopodous structure, but is a retrograde step, analogous to many of those made by De Blainville, which have left only a heap of useless terms, tending to encumber science, until finally swept away into the bin of oblivion.

The figure of the general archetype, or common plan of the Mollusca, e.g. in the Philosophical Transactions of 1853, pl. v., gives a straight intestine with opposite orifices, the anus being terminal at what would be the caudal end of the body in Articulata and Vertebrata. But the most common or archetypal characteristic of the intestine in the Mollusca is its being bent upon itself; with the anus more or less directed towards, and in proximity with, the mouth.

A straight intestine, with opposite terminal outlets, is more common or archetypal to animals generally; but it is just the reflected deviation from this general animal plan of intestine that particularly typifies the molluscous province. A bent intestine does not imply an excessive development of gut; that of the Ascidian and of the Cephalopod is shorter proportionally to the body than is the straight gut of the articulate worm or Crustacean.

Any one who may dissect the common cuttle-fish (Sepia), or squid (Loligo), will appreciate the remark, that it is not because of any excessive development that the gut is bent and the anus directed towards the mouth; it must have been more developed in length to reach the caudal end of the body. But such disposition is not typical of the Mollusca generally.

When the archetype of a group of animals is proposed to show or embody "all the organs and parts which are found in the group, in such a relative position as they would have if none had attained an excessive development," the student requires the standard according to which the "excess" is to be judged. If the proposer of the archetype has no standard to give, his definition has no value.

Another retrograde step in quest of a sound knowledge of the Mollusca has been the borrowing of well-defined terms, previously restricted to exemplify the vertebrate and articulate archetypes, and their application to illustrate the proposed molluscous one. Thus, supposing the alimentary canal, from the mouth to the bend of the intestine, to indicate the division of the body into a part above and a part below such alimentary canal in the slug or snail creeping on level ground, that part in which the functionally most important mass of the nervous system is developed—the supersophageal brain or ganglionic mass, which, by its connection with the optic, gustatory, and sometimes acoustic nerves, bears the closest analogy with the fish's brain—has been proposed to be called the "haemal" side of the slug, and the opposite or under part the "neural" one.

No doubt the upper part of a slug, in which the eyes and feelers, or special organs of exploration, together with the part most like, in function, the pros- and mes-encephalon of fishes, may not be strictly homologous with the dorsal or neural surface of the fish. Such surface in the slug bears a different relation to the vent, the breathing organ, the heart, and, whilst embryonic, to the vitelline or vesicula umbilicalis. But in animals developed on plans so different as the vertebrate and the molluscous, it may be doubtful whether strict homology can be predicated of any organ or part of the body in the two provinces.

What we call coe, the "heart," of a snail, is similar in some degree to that of the fish; it has a similar muscular and valvular bilocular structure, and in both it serves for the propulsion of blood. But in the snail the auricle receives the blood from the breathing organ (lungs), and the ventricle propels it over the body; whilst in the fish the auricle receives the blood from the body generally, and the ventricle propels it to the breathing organ (gills). The parts are to a certain extent "analogous organs," and have thereby received the same name, "heart." The circumstances attending the development of the heart, respectively, in the Mollusk and Fish, equally forbid our regarding such heart as strictly "homologous" in the two types. If such homology had existed in nature, there would then be reason in the assertion, that because the heart is situated on the upper or "brain" side of the snail, instead of on the lower or "belly" side of the body, as in the fish, that therefore such upper side in the snail is the true "haemal" side, and that the snail is as a vertebrate with its "haemal" or "belly" side upwards.

But the heart of the snail is not the same organ, in the homological sense, as the heart of the fish; it does not therefore determine the same side of the body. It might be less inaccurate to regard it as homologous with the pulsating lymphatic heart which is situated on the dorsal side of the vertebral column in some fishes and batrachians.

An equally retrograde step, as breeding confusion by false analogies and by forced and arbitrary double applications of previously well-defined and well-understood terms, is that proposition which would borrow those of "abdomen" and "post-abdomen" from the articulate archetypal nomenclature, and apply them to parts of the diagrams illustrative of the archetype of the Mollusca.

In the Essay alluded to, the "abdomen" of the Mollusk is defined as "that portion of the haema" (meaning dorsal) "region which lies in front of the anus:" to that which lies behind it the term "post-abdomen" is applied. Let the student cut off so much of a squid or cuttle as lies behind the anus, and perform the same operation on a whelk; let him then compare the parts which will be included in so much of the common visceral cavity as he will in each case have removed. When he finds that in both he has brought away the stomach, the liver, the heart, the chief generative glands, &c., then let him endeavour to decipher the sense of the proposition, that in the Cephaloped "the intestine has a neural flexure in consequence of the development of an abdomen;" whilst in the pectinibranchiate Gastropod "the intestine has a haemal flexure in consequence of the development of a 'post-abdomen.'" If he be successful in comprehending the agreement between these propositions and the nature of the things treated of, he may better appreciate than the present writer such modes of illustrating the molluscous archetype.

Were the position of the vent homologically the same in all Mollusks, or even in all Gastropods, then that orifice would truly define two regions of the body, and the part in front of it might need one name and that behind it another. But when so extremely variable a part of the surface as that in relation with the anal end of the intestine, in a Mollusk, is assumed as a determinate basis for the enunciation of archetypal or homological propositions, the warmest aspirant for the progress of philosophical anatomy must doubt whether it has been made in the right direction by such guidance.

In the actual state of Zoology and Comparative Anatomy the same terms are, without doubt, applied to different things in different provinces of the animal kingdom; but those terms are not such as have been rigorously defined for a special or technical signification. Thus we speak of the wings of insects and the wings of birds, and of the legs of insects as of quadrupeds,—knowing all the while that the wings of the beetle are not homologous with those of the blackbird, any more than its hind legs are homologous with those of a horse.

Severe logic would call for distinct names for things so different in their structure and development; but one may ask, Will it be more successful than it has been in other matters where it is opposed to common sense? There is a wise moderation that knows how to avoid the greater evil, by drawing a line beyond which precision of terms would be pedantic purism.

The back of a beetle may not possess all the relations to the organs within, which the back of a mole does; but its analogical similarity, like that between the wing of the beetle and the wing of the bird, will insure the permanence of the corresponding term.

So also with regard to the "foot" and "belly" of the Gastropod. If indeed anatomy could demonstrate intelligibly and satisfactorily to its cultivators the part in a snail which was truly homologous with the ventral surface, or with the foot of a vertebrate, we should be bound to restrict those terms to the so demonstrated homologous part. But in the absence of such demonstrations, science in no degree halts through the application of the same general terms, in a few instances, to parts which have a general and patent analogy, though not a strict homology, in different provinces of the animal kingdom.

In the present article the end of the body of the Mollusk, at or near which the mouth opens, is the "fore" or "anterior end;" the opposite is the "hind" or "posterior end." Where the nervous system is reduced to one ganglion, its position in regard to the alimentary canal indicates the lower or ventral surface of that canal and of the corresponding side of the body; where another ganglion, in connection with antennal and optic nerves, is developed, it is on or near the opposite side of the alimentary canal to that occupied by the homologue of the first or solitary ganglion, and such superadded—usually called "superesophageal" or "cerebral"—ganglion marks the upper or dorsal surface of the alimentary canal, and of the corresponding side of the body.

The "abdomen" is the common cavity of the body which includes the stomach, intestine, liver, and generative gland; and is distinct from the respiratory cavity, which is usually more or less completely partitioned off from the abdomen.

Special homologies can be determined throughout the molluscous province in regard to the principal centres of the nervous system, to the mouth, to the alimentary canal and vent, to the liver, and some other glands. But it is not so certain that other parts bearing the same name in the molluscous province are truly answerable. The branchiae of the ship-worm (Teredo), for example, are analogous in function, but may not be strictly homologous with the branchiae of the whelk or cuttle-fish; neither can the breathing-tube or "siphon" of the Teredo be proved to be the same part homologically with the siphon of the whelk or the funnel of the squid. There is perhaps more ground for inferring that the muscular mass developed from the under or ventral part of the abdomen in most Bivalves is homologous with, as well as analogous to, the muscular part which bears the same name, "foot," in Gastropods.

But whatever doubt may linger regarding the precise homology of the "foot" of a Bivalve and that of an Univalve Mollusk, there can be none respecting the homology of the foot within the limits of the same class. We may trace it as essentially the same part from species to species, from its most rudimental to its most developed state, and under every modification of form, throughout the acephalous or lamellibranchiate Bivalves; and the like with regard to the foot throughout all its modifications in the cephalous Gastropoda and Pteropoda.

With respect to the Cephalopoda, the chief muscular part of the body forms a conical sheath, containing the retractile mouth or buccal mass, and its more immediate appendages. The inner surface of the sheath is smooth; the outer one gives origin to many parts or processes. When it is found that, in the Gastropods, the acoustic nerves are given off from supersophagial ganglia in one order (Heteropoda), and from subsophagial ganglia in another (Pulmonata), the homology of any of the cephalic muscular developments of the Cephalopods with the foot, or part of the foot, of the Gastropods, will not be established by a similarity in the origin of their nerves; but there will be no misgiving as to the cephalic sheath and its appendages being homologous parts within the limits of the cephalopodous class.

The more special definitions of such homology is a legitimate and useful subject of research. When, in reference to the numerous sheathed tentacula which project from the muscular cephalic cone of the nautilus, it is suggested that the sheath is a much-developed sucker (acetabulum), and that the tentacle is the homologue of the caruncle of the sucker on the arms of the dibranchiate Cephalopod, one mode of expressing a sense of the homology of the parts throughout the cephalopodous class is given. According to this view, the anterior circumference of the oral sheath in the nautilus represents four of the eight arms developed therefrom in the cuttle-fish, the four other arms being represented by the four groups of tentacula which are included within the oral sheath in the nautilus.

To this view, while admitting that the oral sheath and its appendages are homologous parts in all Cephalopods, it has been objected that the origin and connection of the inner groups of tentacula in the nautilus correspond better with those of the two long superadded tentacula than with any of the eight ordinary arms in the cuttle-fish, and that each tentacle corresponds, in its complex organization and in the special modification of its inner surface, rather with an entire arm of the dibranchiate Cephalopod than with the caruncle of one of its suckers. According to this view, the more numerous, though comparatively small, tentacles of the nautilus illustrate, in comparison with the fewer and larger tentacles or arms of the dibranchiate Cephalopod, the principle of vegetative repetition. The reduction of the cephalic appendages in number as the Mollusk rises in the scale of the class, their increase in size, and their per-

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1 Valenciennes, Nouvelles Recherches sur la Nautil Flambé, Archives du Muséum, 4to, 1839. 2 Owen, on the Structure and Homologies of the Cephalic Tentacles in the Pearly Nautilus, Annals of Natural History, vol. xii., 1843. The pair of fin-like musculo-dermal organs developed from the sides of the body in Sepiola are analogous to those in Pneumodermus; and if homology could be predicated of musculo-dermal developments in the two classes, the fins of the Sepiola, rather than the funnel, would so answer to the fins of the Pteropod. The disc beneath the part of the abdominal integument supporting and inclosing the shell in Spirula would seem, in like manner, to be more homologous than the cephalic arms with the ventral disc called "foot" in the Carinaria. The derivation of acoustic nerves from the suboesophageal part of the brain inclosed in the cartilaginous skull of the cuttle-fish is as little proof of the homology of such part with the pedal ganglion of a Gastropod as with the supersophageal ganglion of a Heteropod. No homology will stand or carry conviction, the predication of which involves a suppression or a forced explaining away of opposing facts.

If the homology of the muscular cephalic cone and its appendages in Cephalopods with the muscular ventral disc of Gastropods, cannot therefore be demonstrated; and if the predication of special homologies can only be safely and intelligibly made within the limits of the class, not extended from one molluscous class to another, to the degree in which homologous parts can be traced in the Vertebrata, it is owing to the very nature of the classes of the Mollusca. They are more distinct from one another than are the classes of the Vertebrata; they exhibit more different, more strongly marked, modifications of the molluscous organization. Above all, differences of structure in the Mollusca, and especially the developments of dermo-muscular parts, are, in a much greater degree than in the Vertebrata, subordinated to the principle of the conditions of existence, of the reciprocal convenience of parts, of their fitness for the mode and medium of life of the molluscous animal; in a word, are more obedient to teleological than to homological laws.

Every history has its retrogressive as well as its progressive phases; the former have necessitated the foregoing comments.

In resuming the brief account of the real advance of Malacology in recent times, mention ought first to be made of the valuable and beautiful illustrations of the forms, colours, and structure of the new and rare Mollusca collected by the scientific naturalists Péron and Lesueur, Quoy and Gaimard, Eydoux and Souleyet, respectively attached to the circumnavigatory voyages instituted by the French government under Baudin, D'Urville, Freycinet, and Vaillant. Their example has been ably followed by Messrs Adams and L. Reeve in the Mollusca of the Voyage of H.M.S. Samarang, 4to, 1848-1850.

The figures of the molluscous animals given in these and other original works have been accurately copied, and published in a most useful compendium, in 4 vols. 8vo, by Mrs Gray, the accomplished wife of the learned and experienced Keeper of the Zoology in the British Museum.

Baron Férussac has enriched Malacology with a beautiful work in folio, with coloured plates, entitled Mollusques Terrestres et Fluviatiles, of which the last published part, by the Baron, in conjunction with M. Alcide D'Orbigny, treats of the Cephalopods.

Signor Verany has commenced a splendid work, worthy to be regarded as a continuation of that by Férussac and D'Orbigny, on the Mediterranean Mollusca. The first part includes upwards of forty plates of Cephalopods, carefully coloured after the living or recently caught animals.

A work of equal beauty and merit on British Nudibranchiata, by MM. Alder and Hancock, has been published by the Ray Society.

Our accomplished countryman, W. J. Broderip, Esq., F.R.S., lately retired from his arduous duties as police magistrate at Westminster, has taken a praiseworthy share in the advancement of his favourite science, by devoting his scanty leisure to an extensive series of most exact and classical descriptions of new species of Mollusca and their shells, chiefly collected by Mr H. Cumming, and published in the Transactions and Proceedings of the Zoological Society of London. His purse was as liberally opened to secure the rarest specimens of shells brought to the port of London, up to the period when his well-known and most instructive collection was purchased by the British Museum. The several valuable articles on Malacology and Conchology, in the Penny Cyclopaedia are also from the pen of Mr Broderip.

The names of James de C. Sowerby, and of George B. Sowerby, will always be favourably associated with the progress of the molluscous department of natural history. Besides numerous monographs by these authors, the serial works, entitled The Genera of Recent and Fossil Shells, the Species Conchyliorum, the Mineral Conchology of Great Britain, the Malacological and Conchological Magazine, and the Conchological Illustrations, are indispensable to whoever may devote himself to the study and collection of Mollusca and their shells.

M. L. C. Kiener has beautifully illustrated the conchological treasures of Lamarck and of M. Delessert in his Species General et Iconographie des Coquilles Vivantes. Mr Lovell Reeve has performed a similar desirable work in regard to rarities in English conchological collections, and to Conchology in general, in his Conchologia Systematica, Conchologia Iconica, and Elements of Conchology. He is also the author of numerous valuable monographs on subjects of the molluscous province.

The intimate structure of shell has been elaborately and extensively illustrated by Dr Carpenter, Mr Bowerbank, and Professor Quekett.*

Professor Agassiz has, with his characteristic originality, lent valuable aid to the study of recent and fossil shells by his Memoir sur les Moules de Mollusques, Vivans et Fossils, in which the markings of the interior of shells are instructively displayed. The Études Critiques sur les Mollusques Fossiles, by the same author, will be found most useful to whoever may be engaged in the elucidation of Geology by Conchology.

The study of the collection of Mollusca and their shells in the British Museum is now greatly facilitated by the catalogues of that department which have been drawn up or edited by Dr J. E. Gray, F.R.S.*

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1 Voyage de découvertes aux Terres Australes, 4to, 1800-1807. 2 Zoologie du Voyage de l'Uranie, 8vo and fol., 1824-1827; and Zoologie du Voyage de l'Astrolabe, 8vo and fol., 1830-1833. 3 Zoologie du Voyage de la Bonite, 8vo and fol., 1841-1852. 4 Figures of Molluscous Animals, selected from various authors, 8vo, Longmans, 1842-1850. 5 Mollusques Méditerranéens, 4to, 1re Partie, "Cephalopodes," 1851. 6 On the Microscopic Structure of Shells, Trans. of the Microscopical Society, vol. I., 1844. 7 Observations on the Structure of Shells, Trans. of the Microscopical Society, vol. I., 1844. 8 Guide to the Systematic Distribution of Mollusca, part i., by Dr Gray, F.R.S.; Catalogue of the Cephalopoda antepedia, of the Pteropoda of Pinnariidae and Anomiidae, and of Brachiopoda Angulopoda, by Dr Gray; of the Pulmonata or Air-breathing Mollusca, by Dr L. Pfeiffer; of the Terrestrial Operculated Mollusca, by Dr L. Pfeiffer; of the Conchifera or Bivalve Shells, by M. Deshayes; List of British Mollusca and Shells, by Dr Gray, F.R.S.; Nomenclature of Mollusca, by Dr W. Baird, F.L.S.; Catalogue of Molluscan Shells, by P. T. Carpenter. The more intimate knowledge of the nature and affinities of the Mollusca has of late years been signalized by the series of researches on the embryology or development of those animals, more especially those by Rathke, Carus, Sars, Dumortier, Van Beneden, Milne Edwards, Lovén, Windischmann, Koren and Danielssen, Gegenbaur, Schmidt, Vogt, and Quatrefages.

The knowledge of the varying forms of the living Mollusks, of their habits and powers, has been increased, and is likely to be materially advanced, by the rapidly extending practice of preserving them in confined spaces of sea or fresh water. Pali, Montagu, and before them probably other lovers of nature, resident near the sea, availed themselves of large vessels to keep alive, in frequently renewed sea-water, the marine animals in the study of which they were interested. But to Madame Jeanette Power (née de Villepreux), according to the testimony of Professor Carmelo Maraviglia, in the Journal du Cabinet Littéraire de l'Académie Giennia, of Catania, for December 1834, ought to be attributed, if to any one individual, the invention and systematic application of the receptacles now called Aquaria, to the study of marine, and principally of molluscous animals.

Madame Power invented three kinds: one of glass, for preserving and studying living Mollusca in a room; another, also of glass, for small Mollusks, protected by an external cage of bars, in which they could be kept submerged in the sea, and withdrawn at will for inspection; and a third kind of cage for larger Mollusks, which could be sunk and anchored at a given depth in the sea, and raised, when required, for the purpose of observation and experiment. With these different kinds of molluscous menageries, of which the first answers to our present improved and enlarged aquaria, Madame Power carried on her observations and experiments from the year 1832 to 1842 at Messina in Sicily.

She determined the question of the true relation of the Argonauta, or Paper Nautilus, to the delicate boat-like shell which it inhabits. She first showed that the so-called "sails" were normally applied over the exterior of the shell, and proved experimentally that they were the organs which formed and repaired the shell. She proved that the Bulla lignaria preyed upon, and by its strong gizzard ground down and digested, the Dentalium entale. She described the curious manoeuvres by which the Astreopetra aurantiaca seized and conveyed to its mouth and stomach small Natica. And many other interesting facts were brought to light by this persevering and ingenious observer, through the application of the "Gabioline alla Power," as her aquaria were termed by the Giennian Academy, some years before the practice of so studying aquatic animals was introduced and diffused in this country.

The scale on which our improved manufactures of plate-glass have enabled the Zoological Society of London to exhibit living specimens of rare and beautiful marine and fresh-water animals, has struck with admiration the thousands who have witnessed them in the gardens of the Regent's Park.

The management of such aquaria is ably treated of by Mr Warington in the Annals of Natural History for 1855, and the instruction and amusement to be derived from them have been illustrated in a most agreeable and interesting style, in popular works, by Messrs Gosse and Sowerby.

The series of works from the establishment of Mr V. Voors, illustrative of the Fauna of Great Britain, has been enriched by most valuable volumes on the British Mollusca by Professor E. Forbes and Mr Hanley.

Dr Johnston and Mr Woodward have compiled most instructive elementary works on general Malacology; both characterized, and the latter more especially, by careful and original research.

Of the illustrations of particular groups of Mollusca in circumscribed localities, those devoted to the fresh-water family Naiades, and especially to the genus Unio, of the North American rivers, by Mr Isaac Lea, possess the highest value.

Few individuals have contributed so much to the progress of Malacology, since the demise of Cuvier, as Mr Hugh Cuming, F.L.S. His famous collection of shells is known throughout Europe; it contained in 1848 upwards of 19,000 species and well-marked varieties of shells, represented by about 60,000 specimens, which are not only entire, but perfect of their kind, as respects form, texture, colour, and other characters that give the shell value in the eyes of the collector. The mode in which Mr Cuming has accumulated this surpassingly rich illustration of Conchology is as rare and exemplary as the result is marvellous, considered as the work of one individual. Not restricting his pursuit of shells to the shops of the commercia naturalist, the stores of the curiosity-mongers of our seaports, or the casual opportunities of obtaining varieties by purchase, he has devoted more than thirty years of his life in arduous and perilous personal exertions—dredging, diving, wading, wandering, under the equator, and thence to the temperate zones, both north and south,—in the Atlantic, in the Pacific, in the Indian Ocean, and in the islands of its rich Archipelago,—in the labour of collecting from their native seas, shores, lakes, rivers, and forests, the marine, fluviatile, and terrestrial Mollusks, 60,000 of whose shelly skeletons, external and internal, are accumulated in orderly series in the cabinets which now test the strength of the floors of his house in Gower Street, London.

The result of this personal capture of the chief bulk of his collection is, that he has been enabled to assign to each

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1 Om Dammoullusken, Skrifter af Natur-Historie Selhabet, Kjøbenhavn, 1797. 2 Neue Untersuchungen über die Entwicklungsgeschichte unserer Flussmuschel, Nova Acta Acad. Nat. Cur., tom. xvi., 1832. 3 Zur Entwicklungsgeschichte der Mollusken und Zoophyten, Wiegmann's Archiv fur Naturgesch., 1837-1840. 4 Sur les évolutions de l'Embryon dans les Mollusques Gastropodes, Nouveaux Mémoires de l'Acad. Roy. de Bruxelles, tom. x., 1837. 5 Recherches sur l'Embryogénie des Ascidies simples, 1847; and Recherches sur le Développement des Aplyties, 1841. 6 Observations sur les Ascidians Composées, 1840. 7 Ueber die Entwicklung der Mollusca Acephala, Wiegmann's Archiv fur Naturgesch., 1840. 8 Recherches sur l'Embryologie des Limaces (with Van Beneden), Müller's Archiv fur Physiologie, 1841. 9 Bidrag til Pectinibranchierens Udviklingshistorie, 8vo, 1851-1852. 10 Beiträge zur Entwicklungsgeschichte der Land Gastropoden, 1852. 11 Ueber Entwicklung von Limas agrestis, 1851. 12 Recherches sur l'Embryogénie des Mollusques Gastropodes, Annales des Sciences Nat., 1846. 13 Sur la vie intrabranchiale des petites anodontes; et Mémoire sur l'Embryogénie des Planorbès et des Limaces, Annales des Sciences Nat., 1846 and 1841. 14 Raggionglio di osservazioni ed esperienze fatte sullo Argonauta Argo (L.) da Madama Jeanette Power, Prof. C. del Maraviglia, Messina, 8vo, 1836. See also Acti Academici Cataniae, 1838. 15 Relazione per l'anno 1835, dall' Accademia Giennia, Catania, 1837; Abstract of Osservazioni fatte sopra il polpo dell' Argonauta Argo, Lettre nella tornata de 28 Novembre 1835, p. 23. 16 An Introduction to Conchology; or, Elements of the Natural History of the Molluscous Animals. 17 Manual of the Mollusca, 12mo, 1851-1856. shell, not only its country or "habitat" in the ordinary zoological sense, but all the circumstances in which it lived and was developed. If a land-shell, e.g., its favourite rock, or herb, or tree; if a water-shell, the kind of water; and if marine, the habitual depth, and the nature of the sea-bottom at which the Mollusk resided, the rock that it bored, and the animals, the weeds, or other substances it devoured. The value of every shell, in other collections, to which this information applied, became thus greatly enhanced. The importance of these particulars every naturalist and geologist will appreciate on account of the insight that they afford, not only into the economy of living Mollusks, but into the habits and habitats of the fossil shells of the same or allied species; and perhaps one of the most striking points in the estimate of the scientific value of an extensive collection like Mr Cuming's, arises out of its relation, as an indispensable instrument to the determination of fossil shells, to the present active pursuit of Geology.

From the period when the Atlantic, American, and Polynesian departments of the Cumingian collection reached England in 1831, comparative anatomists, malacologists, and conchologists have found subjects, without intermission, for their investigations and descriptions. The organization of the parasitic Stylifer, of Spirula, Calyptraea, Lithodophus, Clavellina, and other rare Mollusks, was brought to light through the specimens which Mr Cuming had preserved in spirits. Almost every part of the Transactions, and every number of the Proceedings of the Zoological Society of London, since the year 1832, contain the descriptions, by Broderip, Sowerby, and Owen, of the novelties liberally submitted to those naturalists by Mr Cuming. And such novelties were far from being exhausted when Mr Cuming, having undertaken a third exploring voyage, returned in 1840 from Manilla, stored with the conchological riches of the Indian Ocean, which have subsequently kept in activity the eyes and pens of Broderip, Sowerby, Pfeiffer, Lovell Reeve, Adams, Dunker, and Philippi.

Of the rare or unique species thus made known to science, Mr Cuming is the possessor, in most instances, of many individuals; and as "dead" shells, or those found without the animals, are strictly excluded from his cabinet, his stores of unique and beautiful specimens give him the command, so to speak, of all the conchological cabinets of Europe. In his annual visits to the Continent, Mr Cuming carries with him the duplicates of his rarities, representing species with the sight of which the eyes of foreign naturalists are gladdened for the first time. They open to him their treasures in return, and from most of the collections—national and private—of Europe Mr Cuming has borne away rare species he did not before possess, in exchange for the still rarer shells which his abundance has enabled him to offer in exchange without detriment to his own rich stores.

His liberality in opening those stores to the student of Conchology equals the enterprise and intelligence with which they have been brought together; and the Cumingian collection is likely long to remain the most important and instructive in Europe for the advancement of the science of the Mollusca.

Mr Van Voorst has brought out, with his usual excellence of type, and more than his usual beauty and costliness of illustration, so much of the Genera of Recent Mollusca, by H. and A. Adams, as relates to Cephalopods and Univalves generally, including descriptions of 680 genera and 437 sub-genera; the number of species nearly 12 to each genus and sub-genus, and altogether exceeds 13,000. The first part of this work was published January 1, 1853.

A considerable proportion of these species are represented by specimens in the Cumingian collection; about half the number are in the British Museum. Besides the shell of each genus, the operculum, when known, is given, and many figures of the living animals have been selected from the best original works.

The classification adopted in this work does not offer any modification based upon original appreciation of structure and affinities overlooked by preceding authors. Inasmuch as the proposed changes are chiefly those of names, and where otherwise, are opposed to deductions from the anatomy of the animals, this part of the work is a retrograde step. Thus the class Cephalopoda is divided into the orders—Octopoda, Decapoda, and Polypoda. Now, besides the inconvenience of the same terminology for a class and its orders, we have here a virtual affirmation that the difference of structure between the Poulp (Octopus) and the Calamary (Loligo) is not less important than that between the Octopus or Loligo and the Nautilus. By the proposed foundation of the orders of the Cephalopoda in 1832 on characters of the respiratory system, Professor Owen sought to harmonize the divisions of the highest class with that of the other classes of Mollusca. The term Polypoda, moreover, is that which has been given by Gis- tel and Aristotle, e.g., to the dibranchiate Cephalopods, or to particular families of them.

In the sale-catalogue of the Yoldi collection of shells by M. Otto Mörch, names that had been applied by old and obscure writers to shells prior to the imposition of generic names on the founding of the Linnaean binomial system, are added, inclosed in brackets, after those names by which the objects are intelligible to Conchologists generally. The Messrs Adams have disfigured their work by the substitution of these justly and practically obsolete terms for many of the well-established generic names applied since the Linnaean system has been in use.

As it may aid in explaining and diffusing the principles upon which the stability of conchological nomenclature is attempted to be preserved by the more judicious cultivators of Zoology, the following list of names offered by the authors of the Genera of Recent Mollusca, as substitutes for the accepted names, is appended, together with the comments of a judicious critic, in which the author of the present article entirely coincides:

| Names in use. | Names proposed by Messrs Adams. | |---------------|--------------------------------| | Hyalea, Lam. 1799 | Carolinella, Glénil (not of Bruguière, 1792) | | Cleodora, Péron, 1810 | Cilo, "Browne" (not of Linn., Müll., Fabr., Brug., Cuv., Lam., Desh., or any other conchologist of note). | | Creoidea, Range, 1828 | Styloidea, Lesueur (testa Blainville). | | Cuvieria, Rang, 1827 | Triptera, Q. & G. 1824. | | Cilo, Linn. 1767 | Chione, Pallas, 1774. |

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1 Descriptions of Shells collected by H. Cuming, Esq., in the Philippine Islands, Zool. Proc., 1840, pp. 83, 94, 119, 155, 189; Ibid., 1841, pp. 22, 34, 36, 44, &c. 2 Ibid., 1840, pp. 87, 96, 116, 135, 167; 1841, pp. 1, 17, 19, 24, 33. 3 Ibid., 1846, pp. 28, 37, 41, 109. 4 Descriptions of 40 new species of Halibota, collected by H. Cuming, Esq., Zool. Proc., 1846, p. 53; Descriptions of 54 new species of Mangonia, collected by do., Ibid., p. 59. 5 Descriptions of new species of Shells, collected by H. Cuming, Esq., Zool. Proc., 1849, p. 169. 6 Diagnoses spectaculorum novarum generis Planorbis Collectionis Cumingiana, Zool. Proc., 1848, p. 40. 7 Descriptions Naticarum quaedam novarum ex Collectione Cumingiana, Zool. Proc., 1851, p. 233. The notes are limited to a few references to the Cumingian memoirs by the above-cited Malacologists, as being sufficient to indicate the richness of the new materials collected by Mr Cuming. 8 Memoir of the Fossil Nautilus, 4to, 1832. 9 This name was given to an imperfect and misunderstood specimen. In the same plate and in the same page, the authors figured and described the perfect Cuvieria under the name of Cleodora obtusa, showing they had no intention of founding a genus (in Triptera) equivalent to Cuvieria. "The question of manuscript names is more difficult, Tunicata, owing to wilfulness of authors. One says it is sufficient to write a new generic name on a tablet and shut it up in his cabinet—it is to be dated from that act." Another distinguished professor of an English university holds that to inscribe the name on a museum specimen is a sufficient act of publication, leaving the determination of the date to the memory of the curator. Some consider the insertion of a new generic name in a catalogue, without a word of description, without even a specific name attached, is sufficient to give 'priority.' Others, more modestly, admit the desirableness of the addition of a known specific name, but do not consider any description necessary; any one that pleases may find out the characters of the new genus, and if it has none, it is but one more name added to the synonymy.

"The genera of Humphrey, quoted in the foregoing list, appeared in the Museum Calceum, a catalogue published anonymously in the year 1797, and containing names only, without definitions. Names attributed to Bolten are also supposed to be taken from a catalogue. We have found the name 'Gevers' placed as the authority for Meuschen's names in the Mus. Gererianum, and 'Berlin' for Link's names in the Berlin Museum. But who wrote the Museum Boltenianum?"

"The Linnaean code, of which Herrmannsen gives an excellent digest, and the rules of the British Association, require that names should be really published, and accompanied by a description sufficient to identify the object and justify the imposition of the new term."

The adoption of generic and specific names in the following pages will be governed by those rules.

PROVINCE MOLLUSCA, Cuvier.

(Heterogangliata, Owen.)

CLASS I.—TUNICATA, Lam.

(Acephales sans Coquilles, Cuvier.)

Acephalous Molluscoids inclosed in an elastic uncalcified tunic, perforated by two apertures, and composed of a peculiar substance resembling the "cellulose" of plants, in having no nitrogen, but only carbon and hydrogen.

ORDER I.—SACCOBRANCHIATA, Owen.

(Les Ascidies and Les Aggregés, Cuv.)

Mantle united to the tunic at the two orifices; elsewhere commonly more or less detached. Branchia, a dilated, ciliated, vascular sac, with commonly a tentaculigerous orifice.

Tribe 1.—AGGREGATA, Cuv.

The aggregate, associated, or compound Ascidians (fig. 5) are all of small size. Their organization is essentially like that of the larger solitary species, but the viscera are somewhat differently disposed,—the cavity of the body is in most longer and narrower, the entire animal viewed singly being more vermiform. In their natural organic association they are arranged in different modes, and under different forms characterizing different families. Some, as the beautiful Diazona, diverge, like the petals of a compound flower, from a common base; others, as the Botryllus, are arranged in circles round a common central aperture.

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1 We cannot find any such "genus" in Guettard's Memoirs, but according to Blainville it was merely a name given to a miscellaneous assemblage, including Vermutes, Scalaria, Magilus, &c.

2 Schmidt, Zur vergleichenden Physiologie der Wirbellosen Thiere, Svo, 1845.

3 Introduction to D'Orbigny's Prodromus de Paléontologie.

4 Greek, signifying "sack-shaped gills." Tunicata, beneath which the anal extremity of the intestine of each individual terminates; whilst many of these circles of individuals are aggregated together, and enveloped in a common cellulose tunic. The substance of this in some species, e.g., *Septodinium*, is crowded with calcareous granules; in others, e.g., *Botryllus*, it exhibits distinct fibres.

Some of the compound Ascidians are ramified (as, e.g., *Perophora*, fig. 5), and their tunics are so transparent as to permit the movements of the internal organs to be studied in the living animal. The individuals of this genus are connected only by tubular prolongations of the common tunic, and are rather "social" than "compound" animals. In these a very singular condition of the circulating system has been detected. The blood moves backwards and forwards, to and from the heart, in the same vessels, as it was supposed to ebb and flow in the human veins before Harvey's great discovery. The oscillation of the currents is not constant and regular; the blood is received from the vessel at one end of the heart (fig. 5, n), and propelled by a contractile wave into the vessel at the opposite end. After a true circulation has gone on in this course for a certain period, a change is observed in the course of the peristaltic contractions of the heart; the blood for an instant stagnates in the sinuses and vessels, and then the wave travels in the opposite direction; the heart drives the blood into the vessel from which it had before received it, and the course of the circulation is reversed. In the compound Ascidians the vascular systems of the different individuals anastomose freely with each other. The veins are chiefly in the condition of large lacunar sinuses. The heart and vessels circulate blood, not water; if the vessels, as some contend, had no proper tunics (and their transparency in the living Ascidians renders them, in most of the sinuses, invisible), the sea-water which freely passes from the branchial to the muscular cavities would flow into the so-called intervisceral lacuna were they merely such as they seem.

At first sight it is difficult to conceive how the fixed and compound Ascidians can multiply their race in situations at a distance from that which they themselves occupy.

This difficulty has been removed by MM. Audouin and Milne Edwards, who observed that the young of the compound Ascidians were not only at their origin solitary and free, but possessed the power of swimming rapidly by the aid of the undulatory movements of a long tail (fig. 5, C). They were seen occasionally to attach themselves to the side of the vessel of sea-water containing them, and then to recommence their course, as if to seek a more suitable point of attachment. After two days of free and locomotive life, they finally fixed themselves, and, when detached, remained motionless.

Similar locomotive phenomena are now known to be common to the embryo of many of the lower sedentary animals. In regard to the Ascidians, it has been confirmed by Sars in the *Botrylli* of the coast of Norway; by Sir John Graham Dalyell in a solitary Ascidian of the Firth of Forth; and the embryogeny of the *Cynthia Ampulla* has been well followed out by Professor Van Beneden.

In the genera *Polycitnun* and *Amoroucium*, amongst the compound Ascidians, Milne Edwards has observed that the ovum, whilst still included in the ovarian mass, consists of the small central germinal vesicle, of a granular vitellus, and a vitelline membrane. In the progress of the ovum to the cloacal cavity the yolk acquires a deep yellow colour, the germinal vesicle disappears, and in its place there is a nebulous speck upon the surface of the yolk. The extremity of the yolk opposite to the caudal attachment develops a series of cylindrical productions. Three of them have expanded extremities which increase in length; whilst the other processes diminish, and finally disappear. A spiral filament is continued from the membrane of the vitellus down the centre of the tail. In this state the embryo escapes from the ovum, generally while in the cloaca of the parent, but sometimes after the egg has been expelled from the common central outlet.

The young animal immediately unfolds its tail, and begins to swim like the tadpole of the frog, which it so much resembles in form. The three clavate cephalic processes are the organs by which Milne Edwards believes it effects its final adhesion and settlement. When this has taken place, the tail shrinks, and is usually detached by progressively increasing contraction at its base,—a kind of spontaneous fission.

The sessile and adherent trunk now becomes the seat of an active development. The integument is thickened. The germ-mass becomes elongated and divided by a circular constriction into two unequal parts, which severally open a passage, constituting, the one an oral (fig. 5, g), the other an anal (fig. 5, f) orifice. The subdivided germ-mass, which now begins to be rapidly metamorphosed into the special tissues, also acquires a distinct tunic b, which soon separates itself from the thick and gelatinous external integument a.

The quadrifid orifice of the branchial sac (fig. 5, e) is first formed upon the internal tunic. The contour of the great respiratory pharynx can next be discerned, and the constriction of the sac opposite to the mouth, which indicates the oesophagus (fig. 5, d). About the same time may be seen the outline of the anal orifice (fig. 5, h) upon the internal integument; then the opaque yellow tunics of the dilated stomach (fig. 5, i) and the reflected intestine appear; and below these parts the pulsations of the large transparent vasaform heart (fig. 5, m) render that organ conspicuous. Around each external orifice some mammilloid processes (fig. 5, g') bud out, which first lengthen, and afterwards in some species become lost in the thickening integument. The eye-speck continues for some little time, and is situated in the middle of the nervous collar (fig. 5, k). At the base of the abdomen the opaline concretionary body appears, to which the heart is subsequently attached, and which is provided with vibratile cilia.

The whole of the viscera included by the smooth integument have been observed to rotate in the cavity formed by the thick gelatinous tunic, to which the visceral mass again becomes attached by the adhesion of the muscular tunic at the branchial and anal orifices, and by the establishment of corresponding orifices in the integument.

Savigny was of opinion that the ovum of the compound Ascidian contained the germs of all the individuals composing the characteristic groups in the mature aggregate animal, and that their development was simultaneous. In one sense, doubtless, the ovum contains the germs of all the future individuals developed by gemmation, in so far as a portion of the germ-mass is retained unchanged in the body of the first developed individual; but the cell-progeny of the primary germ-cell constituting that germ-mass are not simultaneously developed, nor does any development begin until the first individual is completed, fixed, and nourished by the action of its proper digestive apparatus. Thus stimulated and strengthened, the second mode of reproduction, namely, that by gemmation, is superinduced upon the young Ascidian (fig. 5, B), after the foregoing development from the impregnated ovum (fig. 5, A). This offers an interesting ana-

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1 Lister, *Philosophical Transactions*, 1834. 2 Zur Entwicklungsgeschichte der Mollusken und Zoophyten, Wiegmann's *Archiv für Naturgeschichte*, 1837 and 1840. 3 Recherches sur l'Embryologie des Ascidies simples, *Mém. de l'Acad. Roy. de Belgique*, 1847. 4 Mémoires sur les Animaux sans Vertèbres, *Svo*, Paris, 1816. Tunicata. logy to the phenomena presented by the polypine-larva of the Medusa. The individuals (fig. 5, B) formed by the gemmation of the primary bud of the young Ascidian (fig. 5, A), instead of being detached, are retained, the process of gemmation being regulated so as to produce the charac- teristic pattern in which the different individuals are grouped in the mature compound animal.

Family I.—Botryllide.

Body fixed; tunics of many individuals fixed together into a mass, in which the individuals are grouped into sys- tems.

Genus Botryllus, Gaertner, 1774.—Body of the indi- vidual not divided into thorax and abdomen; branchial aperture circular, without rays; social systems numerous, with from six to twenty individuals in each, lying nearly parallel to the periphery of the mass; vent remote from the simple branchial aperture.

Sp. Botryllus violaceus.—Greenish-gray, with dark-blue stars, yellow in the centre round the common orifice. On stones and sea-weed near low-water mark. British.

Genus Didemnum, Sav.—Thorax and abdomen dis- tinct; branchial aperture with six equal rays or lobes.

Sp. Didemnum candidum.—Gulf of Suez.

Genus Diazona, Sav.—Compound group sessile, sub- circular, hollow in the middle; individuals arranged in concentric circles; both branchial and anal apertures six- rayed.

Sp. Diazona violacea.—The compound mass resembles an Actinia, and attains a diameter of 6 inches; it is of a beautiful violet colour. Mediterranean.

Genus Polyclinum, Sav.—Divided into thorax or branchial cavity; upper abdomen with the digestive organs, and lower abdomen with the heart and reproductive organs; branchial orifice six-rayed, anal orifice simple.

Sp. Polyclinum constellatum.—Hah. Mauritius.

In the progress of the gemmation by which the com- pound groups of the foregoing family are formed, a bud is first developed in the form of a small tubercle from the abdominal portion of the internal tunic of the young Botryl- loid. The tubercle becomes prolonged, retaining an active circulation in its interior, and is accompanied by a corre- sponding growth of the outer cellulose integument, which becomes clavate. The process then bifurcates; the divi- sions, in like manner, becoming elongated, expanded, and bifurcated at their extremities.

Soon the outline of an Ascidian is sketched in each of these extremities. The primitive connection with the parent is obliterated; but the young individuals remain united together by their common peduncle according to the law which determines their mode of grouping into systems. By the progressive increase of their outer gelatinous integu- ment they coalesce and form the compound mass.

The procreative force of the germ-mass finally exhausts itself in the formation of the male and female organs, in which that force is again mysteriously renewed, under its two forms of the spermatozoon and the germinal vesicle, by the combination of which the reproductive cycle again begins its course.

Family II.—Clavellinidae.

Body fixed; individuals connected by repent tubular prolongations of their common fixed tunics.

Genus Clavelina, Sav.—Body oblong, erect, pe- dunculate; tunic transparent; branchial and anal apertures without rays; branchial sac short, not plicated, without papillae.

Sp. Clavelina borealis, Sav. (Aseidia elevata, Pallas, Tunica. Spicil. Zool. x., tab. i., fig. 16).—Thoracic region marked with coloured lines. Greenland.

Genus Perophora (discovered by Lister, 1834; so named by Wiegmann).—Body compressed, suborbicular, pedunculated, upon a common repent tubular stem; bran- chial sac occupying a great part of the body, with a papil- lose orifice.

Sp. Perophora Listeri (fig. 5).—Shores of Britain.

Tribe 2.—Solitaria.

(Les Ascidies, Cuv.)

The exterior tunic of the solitary Ascidians (fig. 6, a) is a thick gelatinous or coriaceous elastic substance, adhering by its base, or by a long flexible peduncle, to some foreign body, and perforated at the opposite end or at the side by two apertures h and f (fig. 6). The exterior of this tunic is sometimes rough and warty, the inner surface always smooth and lubricious. Microscopically examined, it consists chiefly of a conglomerate of non-nucleated cells like the paren- chyma of Cacti. Chemically analysed, 100 parts of the tissue, free from ash and water, gives, of carbon 45-38, hy- drogen, 6-47; being the same composition as the "cellulose" of plants.

This non-azotized tissue is traversed by large blood- vessels, and towards its inner surface crystals and nuclei are abundant in the clear homogeneous basis. The lining membrane is composed of a layer of polygonal, nucleated, epithelial cells.

The second tunic bb is muscular; it adheres to the outer tunic at the circumference of the two orifices h, f, and is connected to it by blood-vessels at a few other points; elsewhere it is quite free, and the opposed surfaces of the intervening space between the muscular and elastic tunics have the aspect of a serous cavity. Its fine fasciculi of fibres are remarkably distinct, and are arranged in two layers,— the external circular, the internal longitudinal. The fibres or fasciculi of the outer layer are smaller than those of the inner one, and less regularly disposed.

They describe regular circles around the processes lead- ing to the orifices of the tunic. Other fibres of the outer layer pass transversely from one tube to the other. The longitudinal fasciculi radiate from the two orifices, and decussate each other, winding round the bottom of the sac. Deeper again than this layer there is a sphincter surrounding the base of each tube or orifice, from which a third more delicate layer of longitudinal fibres is given off.

Of the two more or less protuberant and stellate apertures in the outer tunic, one (fig. 6, f) leads directly into the mus- cular sac, the other (h) into a wide vascular branchial sac (dd) contained in the muscular one. The entry to the branchial sac is defended by a circle of short tentacles. A portion of the muscular tunic is dissected from the branchial sac at c. The branchial sac is opened, and one-half reflected back- wards, showing the inner surface at dd; the opposite wall of the sac d is dissected away from the alimentary canal ee. The inner surface of the sac dd is marked by parallel and equidistant transverse lines, the interspaces of which are divided into a series of narrow, vertical, perforated, and richly- ciliated compartments; two opposite narrow longitudinal tracts are entire. A groove along that which traverses the larger curvature of the sac leads to the mouth—an orifice (fig. 6, k) near the bottom of the branchial sac. This orifice conducts, by a short oesophageal canal, to the stomach b; this is an oblong cavity with longitudinal folds. The in- testine is disposed in a sigmoid flexure, adheres to the out- side of the branchial d, and the inside of the muscular sac b,

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1 Gr., signifying "a cluster of grapes." 2 Gr., signifying "double couch." 3 Gr., signifying "in circles." 4 Gr., for "many cavities." 5 Gr., for "little staff." 6 Gr., for "sack-bearer." The liver consists of blind follicles produced into tubes which anastomose, surrounding more or less of the intestine as by a network, and ultimately, at least in *Cynthia tuberculata*, communicating with the stomach by a single aperture, from which a groove is continued towards the heart.

The heart is a simple, elongated, vasiform muscle, inclosed in a pericardium attached to the branchial sac, continued at either end into a vessel; the ramifications of one being expended chiefly upon the respiratory organ; those of the other upon the tunics of the body, or speedily expanding into sinuses surrounding the viscera. According to the direction of the circulating currents, the one trunk-vessel will be an artery, the other a vein, and the circulation itself will be pulmonic or systemic.

The nervous system must be first sought for in the interspace between the two openings of the muscular tunic: there is situated a ganglion (fig. 5, h), from which it is not difficult to trace filaments diverging to each aperture of the sac where the circular disposition of the muscular fibres prevails; other branches accompany the longitudinal fibres, and supply the respiratory sac; two contiguous filaments are continued to the oesophageal orifice.

Eight pigmental spots or eye-specks have been detected at the entrance of the respiratory tube, and six, of a deep yellow colour, at the entrance of the anal tube.

In the animal manifesting this organization, which is much richer unquestionably than the amorphous and rugged exterior would seem to promise, the only vital actions obvious to ordinary vision are an occasional ejection of water from the orifices of the tunic by a sudden contraction, succeeded by a slow and gradual expansion of the entire body. Such contractions and expansions, aided by the ciliary currents, and the peristaltic movements of the alimentary, circulating, and secreting tubes, are all the actions which the organic machinery has to perform in the living Ascidian. The respiratory currents of sea-water, with the nutrient molecules in suspension, are introduced by the ciliary action through the branchial orifice h (fig. 6) into the pharyngeal respiratory sac d, from which the oesophagus k selects the appropriate food. The alimentary excretions and the generative products are expelled through the anal outlet f, by the contraction of the muscular tunic b.

In consequence of the space between this and the outer tunic being closed, that tunic accompanies the muscular tunic in its contraction, through the influence of the surrounding pressure; when the muscle ceases to act, the elasticity of the outer coat begins to restore the contractile sac to its former capacity, and the surrounding water flows into its cavity, either directly or by distending the branchial sac.

We shall find other instances of the economizing of muscular force by the substitution of elasticity as we ascend in the survey of the molluscous organization.

Eysenhardt has observed the act of gemmation in a simple Ascidian. In this, as in the compound kinds, gemination commences by the development of a small tubercle from the abdominal portion of the internal tunic of the young Ascidian. Some solitary *Ascidia*—e.g., *Cynthia*—according to my observations, are of distinct sex. In the male a generative gland, commonly dendritic in shape, occupies the concavity of the intestinal fold, and sends a short and simple duct to terminate near the anus. In the female of the *Cynthia tuberculata* there are two ramified ovaria; the ovisacs being appended to the branches of a central stem, passing up the side of the rectum, and extending over one side of the branchial sac. In these the ova in different stages of development may be seen. The sexes are distinct in *Doliolum*; but are stated to be united in most solitary Ascidians, as in the compound forms. The impregnated germ-cell multiplies itself at the expense of the yolk, and incloses that substance by a series of secondary cells. As this process goes on, the yolk, so subdivided and assimilated, takes on a granular surface, each granule or tubercle having its hyaline nucleus. By the coalescence of the peripheral layer of these cells an external membrane is formed, on the exterior of which are oil-like globules. An albuminous fluid is now interposed between the chorion of the egg and the germ-mass. A filamentary body next begins to be formed from a part of the exterior germ-cells forming the basis of the test, which body bends over the visceral mass. This body or process progressively elongates, then uncoils itself, liberating the rest of the test with the visceral mass, and becoming a freely-vibrating locomotive caudal appendage, as in the larva of the compound Ascidian (fig. 5, C). After its attachment it becomes metamorphosed and developed into the animal, whose organization is illustrated in fig. 6.

**FAMILY ASCIDIAD.E.**

Individual solitary, fixed; branchial sac simple or plicate.

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1 Ueber einige merkwürdige Lebenserscheinungen an Ascidien, Nova Acta Acad. Nat. Curios., tom. xi., 1823. Tunicata. Genus Boltenia, Sav.—Body sub-globular, pedunculate; tunic coriaceous; orifices lateral, 4-cleft; branchial sac deeply plicated, with compound tentacles at its orifice.

Sp. Boltenia reniformis, McLeay.—Greenland. This species has been brought up from a depth of 70 fathoms.

Genus Chelyosoma, Broderip.—Body depressed, discoidal, sessile; tunic coriaceous, composed of polygonal plates; orifices closed by six triangular valves.

Sp. Chel. Macleanianum.—Greenland.

Genus Cynthia, Sav.—Tunic coriaceous, sessile; orifices 4-lobed; branchial sac plaited longitudinally, with a tentacular orifice.

Sp. Cynthia papillata, Sav.; Cynthia microcosmus, Cuv.—The latter is esteemed as an article of food in some parts of the Mediterranean.

Genus Ascidia.—Body sessile, covered with a coriaceous or gelatinous tunic; branchial orifice 8-lobed, furnished inside with a circle of simple tentacular filaments; anal orifice 6-lobed; branchial sac not plaited, its meshes papillated.

The Ascidiae vary in length from 1 inch to 5 or 6 inches. The test is pale and semitransparent, the inner tunic orange or crimson, or sometimes marbled with crimson and white; the ocelli are red, or yellow with a central red spot.

Sp. Asc. gelatinosa (fig. 6).

Order II.—Dactylobranchiata, &c. Ow.

(The character of the order is also that of the single family composing it.)

Family Pyrosomidae.

In one (the commonly occurring) state, many individuals form a compound body, swimming freely, with the tunics fused into a cylindrico-conical mass, in which they have a verticillate arrangement. In each individual the apertures of the external covering are opposite and terminal. The branchial sac with two gills, girt anteriorly by a membranous denticulate ring; open posteriorly. All the species are pelagic.

Genus Pyrosoma, Péron.—A connecting link between the Ascidians and Salpians is afforded by certain compound floating gelatinous Tunicata, called, from their phosphorescent or luminous property, Pyrosoma, the individuals of which are permanently aggregated into a compound organic whole, having a definite form like a flattened cylinder. The common tegumentary mass is toughish and semitransparent. The tubercles with which its exterior is covered consist each of one inhalant end of an individual member of the living group; the opposite exhalant end of the individual opens into the cavity of the cylinder. Besides the common envelope, each individual has a distinct tunic or mantle attached at the oral or branchial orifice, at the anal orifice, and also to the two rounded bodies at the upper part of the branchiae. The mantle is connected with the envelope by the delicate membrane of the venous sinuses. The branchiae are two in number, oval in form, with their dorsal borders in contact and attached to the mantle, and their ventral borders separated by a large sinus; their numerous vessels anastomose at right angles, forming a quadrangular network, and the covering tissue is beset with vibratile cilia, which perform vortex-like movements with beautiful harmony and rapidity. The currents so discharged from the individual into the common cavity produce an outflow from the open end of the cylinder, and the re-action impels it slowly with its blunt end foremost. The oesophagus is curved and of a bright Tunicata. red colour; the stomach is subglobular, yellowish, and opaque; the intestine is short, bent abruptly on itself; the anus directed backwards towards the posterior orifice. The liver is a globular gland, with converging segments; it is attached to the intestinal loop. These viscera are situated posterior to the branchial sac, leaving a free passage to the water, which traverses the cavity. The nerve-ganglion lies upon the anterior end of the branchial sac. The heart is placed at the posterior part of the body, below the visceral mass, which is connected with the mantle by a pedicle near the inner part of the endostyle. There is a long, apparently hollow, filament contained in the dorsal sinus. Savigny figures an ovum, and a part which he supposed to be an oviduct; but the precise condition of the generative organs in the Pyrosoma is at present not clearly made out. Savigny, to whom the knowledge of the compound nature of the Pyrosoma is due, seems plainly to have observed an embryo, which had already constituted by gemmation four individuals before being excluded. This is the commencement of the cylinder at its smaller and closed end; it appears to be elongated by the like parthenogenetic propagation of successive circles or whirls of Ascidian-like individuals.

Sp. 1. Pyrosoma atlanticum, Péron.—Individuals arranged round the cylinder in close whirls. The name was applied to the compound cylinder, under the impression that it was the individual. It is ordinarily from 3 to 7 inches in length, and from 1 to 3 inches in circumference; open at one end, closed and bluntly rounded off at the other.

Syn. Pyrosoma giganteum, Lesueur (Voyage aux Terres Australes, pl. 30, fig. 1); Savigny (Mém., ii., pl. 4, fig. 7; pl. 22 and 23).

The species is widely distributed over the tropical and warmer seas. Viewed from a ship's deck, they sometimes seem to form vast tracts of luminous matter stretched across the waves; when closely scanned, as the keel ploughs through the mass, it appears to be composed of countless cylinders resembling incandescent iron. Fishes and other contiguous marine animals are made visible by the light of the Pyrosoma. It is of a vivid greenish hue, appearing to be emitted in sparks from the individuals in quick succession, beginning at the point touched. In captivity the power gradually fails, and the light excited by a touch or concussion soon fades whilst the animal is in sea-water; but under the stimulus of fresh water it is said to be brightly emitted as long as life remains.

Sp. 2. Pyrosoma elegans, Lesueur.—Individuals arranged round the cylindroid in regular circles.

This species is founded on conical cylindroids of a small size, found in the Mediterranean. (The truth and constancy of the characters of this species need confirmation.)

Order III.—Tæniobranchiata, &c. Ow.

Mantle adhering throughout to the tunic; orifices opposite, terminal or subterminal without tentacles; branchia ribbon-shaped; animals pelagic, free, floating; inhaling water by one aperture, expelling it by the other; individuals, in one generation, connected chain-wise; free in the next generation; and so alternately associated and solitary.

Family Salpidae.

The third order of the Tunicata includes the Salpians, which float in the sea, and are characterized by their transparent elastic outer tunic, which is elongated, compressed,

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1 After Dr Bolten. 2 Gr. for "tortoise-body." 3 Gr., signifying "gills with a ring." 4 Gr., salpa, a luminous fish. 5 Gr., for "fire-body." 6 A name of Diana. 7 Gr., for "a little skin-bottle." 8 Gr., for "riband-gilled." Tunicata, or prismatice-cylindroid, and open at both extremities. The efferent orifice (fig. 7, d) is simple and tubular; it can be closed by a sphincter when the outer tunic expands, the water then flowing in by the opposite orifice e. This is in the form of a transverse slit, having an upper and a lower lip. The latter is reflected inwards, and forms a semilunar valve, allowing the entry and preventing the exit of water; the muscular fibres of the mantle f, or membrane lining the cartilaginous tunic ab, are arranged in flattened, often subannular, bands g; their elementary fibre is striated. The mouth, the stomach l, the liver and the heart o, are aggregated in a small mass or nucleus near the anterior aperture of the tunic; the intestine k extends towards the opposite aperture, and terminates freely at l in the common cavity of the mantle. The intestine, in Salpa gibbosa, has two caeca which project into the centre of the loop. In Salpa zonaria the liver envelopes nearly all the intestinal canal, and consists of a mass of cecal tubes, with a few small appendages near their free ends. The group of small yellowish nervous ganglia is situated just above the oral attachment of the branchia n; near it is often observed a pigment-speck or ocellus. The single narrow plicated riband-shaped branchia n extends obliquely lengthwise across the pallial cavity. The heart o is elongated, and in some species slightly curved and sacculated; it communicates with a large vessel m at each extremity, one of which is ramified principally upon the visceral mass, the other upon the branchia and the muscular tunics. The blood passes into extensive venous sinuses before returning to the heart; its course is oscillatory, as in the Ascidians.

The sexes are distinct in certain individuals of the Salpans, as they are in some of the solitary Ascidians.

The testis is described by Krohn as of an oblong form, single, lodged in the centre of the nucleus in Salpa maxima, in which it consists of numerous delicate seminiferous tubes filled with a white fluid, and opening by a short canal into the common cavity of the body. The two oblong bodies d, sometimes of a violet colour, attached to the mantle, have been regarded as ovaria; but they are more properly a "proliferous stolon," engendering by independent force, or independently of direct impregnation. Thus Sars maintains that the solitary individuals of the Salpae are sexless. The germinative tube, in which the chain of young Salpae are contained, winds round the visceral nucleus, hanging freely by one end in the cavity of the mantle, and being attached by the other end to the back of the nucleus. The Salpae, thus developed from internal buds, are usually produced in groups, which are successively excluded through an aperture in the tunic opposite the end of the stolon.

The chief conspicuous vital action in the Salpans is the rhythmical contraction and expansion of the body, in which the elasticity of the outer tunic antagonises the muscular contraction of the inner one. During expansion the sea-water enters by the aperture e, and is expelled in contraction by the opposite one d, its exit by the first aperture being prevented by the valve. The re-action of the jet, which is commonly forced out of a contracted tube, occasions a retrograde movement of the animal. The currents which successively traverse the interior of the animal renew the oxygenated medium upon the surface of the respiratory organ, bring the nutrient molecules within the reach of the ciliated sub-spiral labial membrane of the mouth, and expel the excrements and the generative products. Thus, a single act of muscular contraction is made subservient to the performance of the functions of locomotion, nutrition, respiration, excretion, and generation.

The aggregate Salpans quit their gemmiparous parent, associated together in long chains. After floating for a certain time, each individual, as De Chamisso first discovered, propagates a young one like itself. The solitary Salpa propagated by each individual of the chain is the product of an impregnated ovum, and is for a time suspended by a peduncle from the dorsal wall of the visceral cavity of the parent. The development of the individual Salpa has been studied by Dr Leuckart. The first evidence of differentiation is the formation in the embryonic mass of a closed cavity, from the walls of which the bar-shaped gill is detached, leaving behind it a space, which becomes the cloaca. The alimentary canal simultaneously makes its appearance. At a later period the branchial chamber communicates with the exterior by the branchial and cloacal orifices. In the Salpa zonaria, however, as many as three pedunculated young have been found in one parent when liberated. The solitary Salpa grows to the size of the grandparent, and then brings forth a social chain of young Salpae, which, by the exercise of their uniparous generation, again give origin to the solitary and multiparous individuals. "Thus," observes Chamisso, "only the alternate generations resemble each other."

The case is strictly analogous to the generation of the compound Ascidians, of which the solitary young gives origin, by generation, to a connected group, each individual of which again procreates, by impregnated ova, free individuals. Most Salpans are usually luminous by night.

Genus Salpa.—The characters of the family are those also of the type-genus. Syn. Thalia, Brown; Daggzya, Banks and Solander (Home, Comparative Anatomy, vol. ii., pl. lxix, figs. 2 and 3, engraved from drawings by John Hunter; see Owen's Physiological Catalogue of the Hunterian Collection, vol. i., p. 133, 4to); Biphores, Cuvier. Sp. 1. Salpa maxima, Forskål. (Syn. Salpa africana, Forsk.)

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1 Observations sur la Génération et le Développement des Elphores, Annales des Sciences Nat., 3ème série, tom. vi., p. 128, 1846. 2 De Animalibus quibusdam e Classe Vermium; Fasciculus I. De Salpa, 4to, 1819. Sp. 2. *Salpa runcinata*, Chamiso. (Syn. *Salpa fusiformis*.)

Sp. 3. *Salpa pinnata*, Forsk. (fig. 7). (Syn. *Salpa cristata*, Cuv.)

Sp. 4. *Salpa gibbosa*, Quoy and Gaimard (*Voyage de l'Astrolabe, Zoologie*, tom. iii., 1835, pp. 569-598, pl. 86-89).

—Specimens, Nos. 480, 481, Physiol. Ser., Hunt. Mus., London.

Sp. 5. *Salpa infundibuliformis*, Quoy and Gaim., ibid.—Specimens, Nos. 482, 483, Physiol. Ser., Hunt. Mus., Lond.

Sp. 6. *Salpa polycretica*, Forsk.

Genus *Anchinea*, Eschricht.—This name has been given to small Salpae, associated in a single row to a gelatinous filament.

Genus *Dolichum*.—Body cask-shaped, open at the ends; muscular bands six, equidistant; branchiae in two bands with vertical bars, one on the dorsal, the other on the ventral surface, converging and confluent posteriorly.

Sp. *Dolichum denticulation*, Quoy and Gaimard.—Only the solitary form of this species has been yet observed.

Genus *Appendicularia*, Chamiso. (Syn. *Oikopleura*, Mertens; *Vezzaria*, Müller.)—Body ovoid, with a long appendage from the dorsal surface of the posterior end, making an angle with the axis of the body, and leading to a large cavity with venous sinuses. The branchia is represented by a ciliated pharynx, communicating with the exterior by two short tubes (discovered by Gegenbauer), ciliated where they leave the cavity, and opening immediately on the dorsal surface in front of the base of the appendage.

Sp. *Appendicularia flabellum*, Chamiso.—This species is one-sixth to one-fourth of an inch in length, and has been observed in such numbers as to form cloudy patches of red substance on the surface of the sea. It resembles the larva of the Ascidians; but Mr Huxley states that he observed spermatozoa in some individuals. No female or aggregated form of *Appendicularia* has yet been seen.

CLASS II.—BRACHIOPODA, Cuvier.

The Acephalous Mollusks of this class are deprived, like the Ascidians, of the power of locomotion, and are attached by a peduncle or by the shell to foreign bodies. Their soft tunic or mantle is, as it were, slit open, and consists of two broad membranous expansions, called "lobes," which are covered by two calcareous plates or "valves," applied, one to the dorsal, the other to the ventral surface, so as to inclose and defend all the soft parts of the animal. The breathing organs consist chiefly of vascular ramifications or processes on the inner surface of the pallial lobes, whence the name Palliobranchiata, which has been applied to the Brachiopods. The viscera are situated at the part of the shell next the hinge or peduncle, and in one order are confined to a small space there. The rest of the interspace of the pallial lobes is almost entirely occupied by two long fringed arms, continued from the sides of the mouth, and disposed wholly or partly in spiral curves. All the species are marine. They may be briefly characterized as follows:

Body symmetrical, with a dorso-ventral bivalve shell, lined by a closely-adherent mantle of two widely-separated lobes. Oral branchia two, long, fringed, and ciliated, usually more or less spirally disposed.

ORDER I.—ARTHROPOMATA, Ow.

Shell-valves articulated; calcareous; viscera occupying one-third, branchia two-thirds, of the shell-cavity.

Family I.—Terebratulidae.

Shell-structure minutely tubular. Ventral valve (fig. 8, V) with a prominent notched or perforated beak, and two curved hinge-teeth; dorsal valve (fig. 8, D) with a depressed umbo, a cardinal process between the dental sockets, and an internal calcareous, usually looped, appendage supporting the two long bent and spiral branchia d, e; attached by a short pedicle n, or sessile by a part of the ventral valve.

The subdivisions of the Terebratulidae or lamp-shells, which include the most characteristic living forms of the Brachiopod class, are based upon modifications of the brachial appendage. The foramen in the ventral valve is usually more or less formed by a small separate shelly piece called the deltidium.

The viscera and shell-muscles occupy a small space near the hinge; the rest of the shell-cavity is almost wholly occupied by the two long fringed arms, which diverge from the sides of the mouth and terminate in spiral coils (fig. 9, f).

Of the muscular system, the adductor longus anticus is shown at o (fig. 8); the adductor longus posticus at p (figs. 8 and 9); and the adductor brevis at q. Other parts of the complex muscular system are described and figured in Owen's *Anatomy of Terebratula*, Introduction to the British Fossil Brachiopoda, by Davidson, printed for the Palaeontographical Soc. 1853.

The stem of the branchia is hollow (fig. 9, e), and contains muscular fibres so disposed as to compress the fluid contained in the cavity. The nervous system consists of three principal parts—the "pallial," "brachial," and "visceral;" the origins of these systems centre in the ganglionic basis of the resophagical ring, which basis determines the dorsal and ventral aspects of the body as above defined. The details of the nervous system are described and figured in the General Introduction to the British Fossil Brachiopoda, above cited, p. 11, pl. 2; and in Owen's Lectures on Invertebrata, Svo, 1855, p. 491.

The mouth of the Terebratula (fig. 9, a) is situated in the middle line, and opens ventrad above the fringed transverse bar or common base of the branchia, and at the beginning of the passage formed by the spiral fold of the branchia; a short esophagus ascends dorsal, and swells into the stomach, which curves backwards and downwards to b; the major part of the cavity is concealed by the hepatic follicles d, d; the intestine e is short, and terminates near the ventral valve behind the adductor posterior. The writer of the present article has detected fecal masses of infusorial shells in this intestine, and has pressed such out of the intestine in young specimens of Terebratula caput-serpentis, recently dredged; but the anus appears to become obliterated in some mature Terebratula.

The mantle lobes are remarkable for the size of the branched vessels of their inner layer, and for the tubular processes of the outer layer, which penetrate the pores of the shell. The trunks of the pallial vessels are connected with organs (fig. 9, o), discovered in the Terebratula flavescens, and first described as "a small transversely-plated membranous process, continued from each side of the beginning of the intestine." The homologous organs were soon after independently discovered in Lingula anatina by M. Vogt, who described them as "peculiar sacs," lying one upon each heart, with its free border folded like a frill. "At the connecting line of the folds there is a fissure which leads to an extremely delicate canal, whose further continuations I could not follow; but it seemed to me that it opened externally between the two lobes of the mantle."

The present writer, having subsequently convinced himself of the communication of these plicated organs with the visceral sinuses and ramified pallial vessels, described them as having the relation of auricles or hearts to that vascular system. Lovén concludes, from their external orifices, that they are kidneys; Mr Hancock that they are oviducts. The latter author conceives that the pallial vessels circulate, not blood, but sea-water, and compares them to the atrium of the Ascidians, and the water-chambers of the Cephalopods. He confirms the connection of this system of vessels with the plicated organs, the relations and varying conditions as to size of which are figured in the Anatomical Introduction above quoted, pl. 3. The ramification and subdivision of the pallial vessels near the margin of the mantle, the complex fringe of setigerous (Terebratula) or jointed (Lingula) bristles at their free margins, the modifications and developments of the inner layer of the mantle, described by Cuvier as "gills" in Lingula, and the perforating pallial tubuli discovered by Dr Carpenter,—all point to the respiratory function of the pallial lobes. The present writer has, however, observed ciliary actions on the fringes of the branchia in a living Terebratula caput-serpentis; and has remarked that "wherever the blood is exposed in its vessels and sinuses to the sea-water, a respiratory action must go on." A slender filament accompanies the larger pallial vessels throughout all their ramifications. The generative cells (testes or ovaria) are developed within the pallial vessels, and apparently from the more slender ramified body. In a Lingula anatina, preserved in spirits, the writer has observed the following stages of development in ova taken from the ramified pallial ovarium, which was bathed in the fluid of the correspondingly ramified sinuses:—1st, An impregnated ovum, in which the germinal vesicle had disappeared and the germ-mass had been formed; it occupied the entire ovum, which had assumed an oblong form; a peripheral stratum of the derivative germ-cells was more compact and of a somewhat lighter colour than the central mass. 2d, A germ, showing the formation of a smooth membrane round the germ-mass. 3d, A germ with a central cavity and the rudiment of a peduncle. 4th, An embryo with the peduncle more produced, but without a visible trace of the shell. These stages are described and figured in the Anatomical Introduction to the Fossil Brachiopoda, p. 22, pl. 1, fig. 7, a to c. They show that the young Lingulae are provided with a peduncle before they quit the parent; and it is very improbable that they escape through the plicated organs which Mr Hancock regards as oviducts.

The first step in the investigation of a small and complex Mollusk, especially when, as a rarity, it is brought preserved in spirit to the anatomist, is to find, describe, and figure the structures that exist; the determination of their function, when found, is the work of ulterior research, and in regard more especially to the circulation, depends upon observations of the living animals. The present writer has derived much satisfaction in finding almost all the parts and organs which he had described and figured in successive contributions to the anatomy of the Brachiopoda confirmed by later investigators, with a difference of opinion as to the function of some parts, and a change of name of other parts. The anus of the Terebratula, e.g., is described and figured by Mr Albany Hancock in the beautiful drawing of the dissection of the Waldheimia flavescens which he contributed to Dr Gray's Catalogue of the Mollusca Brachiopoda of the British Museum, 12mo, 1853, p. 14, fig. 2, also in the figure published in Mr Woodward's Manual of the Mollusca, p. 210; and in Mr Davidson's Classification of the Brachiopoda, Palaeontographical volume for 1853, p. 55, fig. 1. But in specimens subsequently dissected, Mr Hancock states he has not been able to detect the outlet at the end of the intestine. The same author, while he admits that the walls of the branchial canal (fig. 9, e) are supplied with delicate muscular fibres, which run diagonally round the tube, and which are crossed by "delicate longitudinal fibres," failed to detect anything like the "double spiral arrangements of fibres" described by Professor Owen. Whether science really gains by such changes of nomenclature as of "occlusors" for "adductors," and of "adjustors" for "protractors," may be questioned.

Genus TEREBRATULA (as restricted by Davidson).—Shell oval, elongated, or transverse, externally smooth or pitted; valves more or less unequally convex; margin even or waved; hinge-line curved; beak short, truncated by a foramen, partly margined by a deltidium in one or two pieces; appendicular (internal) loop short, attached to the dorsal valve; branchia projecting considerably in front of the loop; no internal septum in the ventral (socket) valve. Sp. Terebratula vitrea, Glassy Lamp-shell.—Habitat Mediterranean, in 90 to 250 fathoms. More than 100 species of Terebratula with short loops have been found fossil. They range from the Devonian epoch to the present day. Of the fossil species the most remarkable is the Terebratula diphyra of Colonna, from the Oolite of Italy. Both valves are perforated in the mid-line, in the same perpendicular, in advance of the hinge. Mr Woodward states that the young had bi-lobed valves, which united in the adult so far as to leave the above apertures.

Genus TEREBRATULINA, D'Orb.—Shell generally longer than wide, more or less oval, finely striated; deltidium small; in some obsolete loop short, becoming annular in the adult by the union of the oral processes, forming a shelly band behind the mouth.

Sp. *Terebratulina caput-serpentis* (Snake-headed Lampshell).—Hab. from Norway to the Mediterranean, at from 10 to 150 fathoms. About twenty fossil species are known; ranging from the Oolitic to the actual epochs.

**Genus Waldheimia**, King.—Shell smooth or plaited, with the dorsal valve impressed, in some species depressed or concave; perforation of beak completed by a deltidium of one or two pieces; loop long, usually exceeding two-thirds of the length of the valve, and reflected; septum of dorsal valve elongated.

Sp. *Waldheimia flavescens* (*Terebratula australis*, Quoy and Gaim.)—Hab. Australia, from just below low-water mark to 20 fathoms. This is the species of which the anatomy is illustrated in the *Anatomical Introduction to the British Fossil Brachiopoda* above cited, and in fig. 9 of the present article.

*Terebratulidae* with long loops have been found from the Lias to the present period.

**Genus Terebratella**, D'Orb.—Shell smooth, or with radiating folds; dorsal valve longitudinally impressed in most; beak-hole partly margined by a deltidium of two pieces, in some disunited above the umbo; beak with a flattened area on each side of the deltidium; loop elongated and attached to the septum as well as to the hinge.

Sp. *Terebratella dorsata*, Lam.—Hab. Straits of Magellan. The soft parts and apophysary system of the genus were described from this living species in the *Transactions of the Zoolog. Society*, vol. i., p. 141, pl. xxii., 1835. The species of this genus, now ranked under sub-genera, range from the Cretaceous period to the present day.

**Family II.—THECIDIIDÆ.**

The characters of the family are also those of the single genus composing it.

**Genus Thecidium**, De France.—Shell thick, punctate, attached by the substance of the ventral shell at or near the beak; interiorly a deep or thick granulated margin generally encircles the valves, and in the ventral one, after reaching the front, extends inwards in the shape of a rounded mesial ridge; oral processes united, forming a bridge over the visceral cavity; loop often unsymmetrical, lobed, and united more or less intimately with the sides of the grooves.

Sp. *Thecidium mediterraneum*.—Hab. Mediterranean; attached to corals. The animal has elongated brachia folded on themselves, with their curve long and not close-set.

"This genus, so far as we know, first appeared in the salt-marshes of St Cassian. In the Lias the species are both numerous and large; from that epoch the genus continued uninterruptedly to the present period, and the family is still represented by a single species." (Davidson's *Monograph*, above cited.)

**Family III.—SPIRIFERIDÆ**, Sowerby.

All the species are extinct; they were chiefly distinguished by much-developed brachia, supported on spirally-disposed shelly processes, which were sometimes spinulose.

Davidson admits five genera and two sub-sections of this family. The species range from the lower Silurian epoch through all the Palaeozoic formations, and apparently died out in the Lias.

**Family IV.—RHYNCHONELLIDÆ.**

Shell impunctate, oblong, or trigonal beaked; hinge-line curved; no area; valves articulated, convex, often sharply plaited; foramen beneath the beak, usually completed by a deltidium, sometimes concealed; hinge-teeth supported by dental plates; hinge-plate deeply divided, supporting oral lamellae, rarely provided with spiral processes; muscular impressions grouped as in *Terebratula*; vascular impressions consisting of two principal trunks in each valve, narrow, dichotomizing, angular, the principal posterior branches inclosing spaces which leave punctate impressions in casts of the shell.

**Genus Rhynchonella**, Fischer (*Notice des Fossiles du Gour de Moscou*, 1809).—Animal with elongated spiral arms directed inwards towards the concavity of the dorsal valve; visceral mass scarcely extending beyond the umbones, separated from the general cavity of the shell by an aponeurosis, in the centre of which is the mouth; labia united at the sides of the mouth and produced into the spiral branchia; alimentary canal terminating behind the insertion of the adductor in the ventral valve; mantle not adhering, its margin fringed with a few short setæ.

Sp. *Rhynchonella psittacea*.—Hab. northern seas. The organization of this recent species is illustrated in the *Transactions of the Zool. Society*, vol. i., 1835, p. 145.

*Rhynchonella nigricans*.—Hab. New Zealand. The above are the only two existing species at present known. Upwards of 250 species of this family have been found fossil. It manifests one of the most ancient types of animal organization, ranging from the Cambrian to the present period. The chief modifications of the family type were exemplified by the extinct genera *Pentamerus*, Sowerby; *Camarophosia*, King; *Atrypa*, Dalman.

**Family V.—ORTHIDÆ.**

Shell transversely oblong, depressed, rarely foraminated; hinge-line wide and straight; beaks inconspicuous; valves plano-convex, or concavo-convex, each with a hinge-area notched in the centre; ventral valve with prominent teeth; muscular impressions occupying a saucer-shaped cavity with a raised margin; adductor central; cardinal and pedicle impressions conjoined, lateral, fan-like; dorsal valve with a tooth-like cardinal process between two curved brachial processes; adductor impression quadruple; vascular impressions consisting of six principal trunks in the dorsal valve, two in the ventral, the external branches turned outwards and backwards, inclosing wide spaces called ovarian. Indications have been observed in several genera of horizontally-coiled spiral arms, but they had no calcified supports; the space between the valves is often very small. The shell-structure is minutely tubular or punctate, except in a few instances, where the original texture is probably obliterated.

All the species are extinct; some appear to have lived free, others to have been fixed by means of a pedicle. They range from the Cambrian to the end of the Liassic period; the type genus *Orthis* has not been found above the Carboniferous formations.

**Family VI.—PRODUCTIDÆ.**

Shell concavo-convex, with a straight hinge-line; valves in some species (*Stropholosia*, King) regularly articulated, in others without hinge-teeth; closely appressed, striated,—the striae developing close-set tubular spines; ventral valve convex; dorsal valve concave; internal surface dotted with conspicuous funnel-shaped punctures; dorsal valve with a prominent cardinal process; brachial processes sub-central, without calcified supports; vascular markings lateral, broad, and simple; adductor impressions dendritic, separated by a narrow central ridge; ventral valve with a slightly notched hinge-line; adductor scar central, near the umbo; cardinal impressions lateral, striated.

All the species are extinct; they range through the Devonian, Carboniferous, and Permian periods.

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1 Gr. for "small pouch." 2 Gr., sign. "small beak." 3 Gr. for "straight." Order II.—LYOPOMATA, Owen.

Shell-valves inarticulated, and, save in the annectant family Cranidae, subcalcified; viscera occupying one-half; brachia the other half, of the shell cavity.

Family I.—CRANIADAE.

Shell orbicular, hingeless; external surface smooth, or spinous, or with radiating costae or foliaceous expansions; dorsal valve limpet-like; interior of each valve with a broad granulated border, well exhibiting the tubular or punctate shell-structure; disk with four large muscular impressions, and digitated vascular impressions.

Genus Crania, Retzius, 1781.—Animal usually attached by the substance of the ventral valve, with free spiral arms directed towards the concavity of the dorsal valve, and supported by a nose-like prominence in the middle of the lower valve; mantle-lobes extending to the edges of the valves, and closely adhering, with the margins plain and thin.

Sp. Crania anomala, Müller.—The thick, cirrigenous, spiral, labial arms are confined to the anterior half of the shell-cavity, and are devoid of calcareous supports; the pallial sinuses are digitate.

"The valves of Crania appear to have been opened by the action of sliding muscles, as described by Professor Owen in Discina; those which advance the dorsal valve (protractor muscles) are aided by a single small muscle in the median line, answering to the cardinal muscle of Terbratula. The pair of muscles which bring the free valve back to its place (retractors) are attached outside the anterior adductors of the free valve, and to a point between the posterior adductors in the fixed valve, like their equivalents in Discina, according to Owen."

The species of Crania, most of which are now extinct, range from the Cambrian to the present period.

Family II.—DISCINIDAE.

Shell-valves unarticulated, minutely punctate; animal attached by a very short and thick mass of musculo-tendinous fibres, passing through the ventral valve by a slit in the hinder portion, or by a circular foramen.

Genus Discina, Lam., (Orbicula, Broderip and Sowerby).

Sp. Discina (Orbicula) lamellosa, Brod. The following are notes of dissection of two specimens of this species, which were given to the writer by Mr H. Cuming, who collected them on the Peruvian coast:—The shell is of an irregular, rounded, flattened form, of a reddish-brown colour; the edges of the layers of increase are horny, and the margin of the shell is of a soft texture and thickened. The layers of increase are large in proportion to the size of the shell, and very irregular; the dorsal valve resembles that of a low limpet, having the apex behind the centre; the ventral valve is flat. The orifice in the ventral valve is a slit at the bottom of a depression; it is three lines long and half a line wide. Through this slit the organ of adhesion passes, and immediately expands into a round sucker, which fills up the whole of the depression, concealing the margins of the slit. Beyond the margins of the valves long shining setæ project from two to four lines, which are inserted all round the free or disintegrated margins of both lobes of the mantle; but the outer shell-forming layer of the mantle extends a little beyond the setigerous border. The setæ are much longer in proportion than in Terbratula, and rather longer than in Lingula; they are beset with minute setæ. The muscles and viscera are situated in a rounded mass in the posterior half of the shell. At the fore-part are two oblong ends of muscles with their anterior sides converging; in the triangular space between them is the green-coloured liver; behind these the mass of the gray ovary; and at the posterior part of the circle are two smaller "post-adductor" muscles.

On removing the lower valve, which must be cut through to the fissure on each side in order not to disturb the viscera, the same structure of mantle is perceived, there being a single vascular trunk or sinus on either side; but the viscera are quite concealed by the dilated disc for adhesion. On removing the mass of the follicular liver and ovary, the alimentary canal is observable extending down the middle to the posterior part of the shell, and then bending to the right, viewed from behind, it enters the right egg-receptacle, and terminates by opening externally between the mantle-lobes and just behind the right extremity of the arm. Four shining tendons are also observable: two superior diverging. The two superior pairs arise by a common origin from between the insertion of the anterior large muscles, and are inserted outside the posterior pair of muscles; and two inferior converging, to be inserted at the inner side of the same. These are attached by their opposite extremities to the sides of the membranous circle containing the ova, and protecting the viscera. The two posterior large muscles are inserted into the lower valve close to the slit.

On turning down the ventral lobe of the mantle, the vessels are also seen converging from the respiratory margin to two trunks, which terminate in two oblong plicated cavities on either side. These are the hearts of the pallial sinuses ("kidneys" of Lovén, "oviducts" of Hancock); they are situated between the two strong membranes which circumscribe the visceral mass, and divide it into two cavities. These cavities are large, and were filled with a coagulated light-gray substance and a brownish matter; the latter, under the microscope, presented a granular appearance, like ova, but the lighter substance had no form or structure; it might be coagulated blood. The arms are joined at their bases below or across the ventral side of the mouth, on which aspect they form a transverse, semilunar, fleshy stem, fringed and convex anteriorly; at the extremities of this ridge they are bent upon themselves, and turn upwards and forwards their extremities, making one turn and a half in front of the mouth. These reflected parts of the arms are closely connected to the first-described portion, and cannot be freely everted and protruded, as in Lingula. The lateral parts, where the bend is, may be pushed out like elbows, and so the shell be opened; or it may be opened by their contracting from the bend towards the mouth, and so becoming thicker and opening the shell. The brachial fringe consists of round equal filaments, with an entire surface; but when viewed through the lens, dark lines seem to traverse them longitudinally.

The Discinidae, of the genus Trematis, Sharpe (Orbicella, D'Orb.), as well as those of Siphonotreta, De Vernueil, belong to the Silurian periods. The species of Orbicelloidea, D'Orb., range from the Silurian to the Neocomian periods, and were most numerous during the Carboniferous period. The true Discina, as restricted by D'Orbigny, first appeared at the Tertiary epoch, and are still represented by the species which the writer has had the opportunity to anatomize.

Family III.—LINGULIDÆ.

Shell oblong or orbicular, sub-equivalve, attached by a long pedicle passing out between the beaks of the valves; texture horny, minutely tubular; arms fleshy, unsupported by calcified processes.

Genus Lingula, Bruguière, 1789.—Shell oblong, compressed, slightly gaping at each end, truncated in front, rather pointed at the umbones; dorsal valve rather shorter, with a thickened hinge-margin and a raised central ridge inside. Animal with the mantle-lobes firmly adhering to the shell, and united to the epidermis, entire circumference of their margins distinct, and fringed; pallial sinuses giving off numerous narrow loops from their inner surfaces; visceral cavity occupying the posterior half of the shell, and surrounded by a strong muscular sheath; pedicle elongated, thick; adductor muscles three, the posterior pair combined; two pairs of retractors, the posterior pair unsymmetrical, one of them dividing; protractor sliding muscles, two pairs; stomach long and straight, sustained by inflections of the visceral sheath; intestine convoluted dorsally, terminating between the mantle-lobes on the right side; oral arms disposed in about six close whirls, their cavities opening into the prolongation of the visceral sheath in front of the adductors.

Sp. Lingula anatina, Lam.—From dissections of this species, the Brachiopodous type of structure was first made known in the memoir by Cuvier in the Annales du Muséum, vol. i., 1802. Cuvier here shows, in Lingula, a condition of the respiratory organ which might be paralleled with one of the transitory states of that organ in the Lamellibranchs,—that, viz., in which the rudimental gills appear as processes from the inner surface of the pallial lobes, and in which the distinction, whether morphological or physiological, of the gills and mantle is not fully established. The modifications of the breathing organ in both Terebratula and Discina exhibited a more interesting condition, comparable to a still earlier stage of the respiratory system in the embryo Lamellibranch,—that, viz., in which the vessels of the pallial lobes have not begun to bud out in parallel rows of vascular loops,—the first stage in the formation of gills, and the one at which it is arrested in the Lingula. Additional particulars of the organization of Lingula will be found in the Transactions of the Zoological Society, vol. i.; in the Anatomie der Lingula anatina, by Vogt; in the Neuer Denkschrifte der Schweizerischen Gesellschaft, Bd. vii.; and in the Introduction to the Monograph on British Fossil Brachiopoda, pl. i., figs. 5, 6, and 7; pl. ii., fig. 3.

The high antiquity of the Lingula, and the maintenance of the genus under very slightly modified specific representatives to the present day, render a knowledge of so enduring a plan of organization highly interesting. Fossil Lingula (L. Davidii, M'Coy, e.g.) occur in the Cambrian or lowest fossiliferous beds of North Wales; and Lingula continued to exist in the seas that are now British as late as the period of the Coralline Crag. The genus is represented at the present day by living species, which as yet have only been found in warmer latitudes; e.g., in the seas washing the coasts of the Indian Archipelago, the Moluccas, Australia, the Fiji and Sandwich Islands.

The Lingula anatina, living near the surface, and sometimes where it would be left exposed by the retreating tide were it not buried in the sand, must meet with more variety and abundance of nutriment than can be found in the deeper waters, where Terebratula, Crania, and Discina usually reside. Hence its powers of prehension are greater; and Cuvier suspects that it may even enjoy a species of locomotion from the superior length and muscularity of its pedicle. The organization of its mouth and stomach indicates the molecular character of its food; but its convoluted intestine shows a capacity of extracting a quantity of nutriment proportioned to its superior activity, and to the greater extent of its soft parts. The more extended pallio-vascular surface is in harmony with the above conditions of structure and habits.

With regard to Discina, and more especially the deep-sea species of Terebratula, both the respiration and nutrition of such animals—which exist beneath a pressure of from sixty to ninety fathoms of sea-water—are subjects suggestive of interesting reflections, and lead one to contemplate with less surprise the great strength and complexity of some of the minutest parts of the frame of these diminutive creatures. In the unbroken stillness pervading those abysses their existence must depend upon their power of exciting a current around them in order to dissipate the water already laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted to their sustenance.

The Brachiopods appear to have been most abundant, and certainly manifested their most varied forms, in past epochs of the globe. They are now, though comparatively few in number and restricted in generic modifications, widely diffused over the earth's surface; and some species exist at greater depths than other bivalve Mollusks. They are thus amongst the most ancient of existing types of animal structure, and their range in space is as extensive as has been their range in time.

With regard to the affinities of the Brachiopoda, Siebold compares their spiral-fringed brachia to the tentacles of Alcyonella; and this idea of their affinity to the Bryozoa has been reproduced and supported by Mr Hancock and Professor Allman, who deem the affinities of the Bryozoa to pass off in two directions,—in one through the Tunicata, and in the other through the Brachiopoda; but they believe the Brachiopoda to be much more nearly allied to the Lamellibranchiata than they are to the Bryozoa. Cuvier indicated his sense of the distinct and well-marked type of the structure of the Brachiopoda by placing the class below that of the Tunicata,—a position which it retains in the edition of the Regne Animal of 1830.

The knowledge of the structure of the mature forms of Terebratula, Waldheimia, Rhychoenella, Crania, Discina, or Orbicula, and Lingula, led the writer in 1835 to regard the Brachiopoda as a group equivalent to the Tunicata on the one hand, and to the Lamellibranchiata on the other; and in the linear series, necessitated by progressive description, the proper place of the Brachiopoda was deemed to be, as in the present article, between the Tunicata and the Lamellibranchiata. Should the embryology of the Brachiopoda show that they propagate by gemmation as well as by impregnated ova, and that the embryo quits the ovum under the guise of a ciliated gemmule, such facts would weigh strongly for the closer affinity of the Brachiopoda to the ciliobrachiata Polypes. Should the Brachiopoda be found to propagate exclusively by impregnated ova, and—as the scanty observations on Lingula show—that the embryo undergoes a development which brings with it a recognizable approach to the mature form before it quits the parent—should the little embryonal Brachiopodous Bivalves be found also to be hatched in the sinuses of the respiratory layer of the pallial lobes,—all these developmental characters will tend to confirm the position which is here assigned to this well-defined class of Acephalous Mollusks.

CLASS III.—LAMELLIBRANCHIATA.

(Acephales Testacea, Cuvier; Bivalvia, Fleming).

Approximations to the characters of the Lamellibrachiata Bivalves are made in the foregoing class by Rhychoenella, in respect to the structure of the shell, by the absence of the minute canals and co-adapted pallial tubes which traverse the shell in other Brachiopods; in respect to the mantle, by its being consequently less adherent to the valves; and in respect to the oral brachia, by the rudi... mentary condition of their calcareous supports, and by their consequent greater freedom and mobility, in which they resemble the long labial palps of Anomia. A like approximation is made by Lingula in the complexity of the inner respiratory layer of the free lobes of the mantle, and in the proportion of the shell-cavity occupied by the visceral mass. Reciprocally, the genera Anomia and Ostrea, among the Lamellibranchs, resemble the Brachiopoda in the attachment of the shell to foreign bodies by either its substance or by altered muscular fibres, and in one valve being downwards, the other upwards; so that they are distinguished into "lower" and "upper" valves, in the position in which the living animals are commonly found. The lobes of the mantle, moreover, are as little united as in most Brachiopods; and the generative glands are developed in a ramified form upon them in Anomia; in which genus, finally, the muscles of the shell and pedicle or plug present a complexity almost equal to those in Discina.

The homology of the "pallial lobes" of Brachiopoda with the same-named parts in Lamellibranchiata, and of the mouth and its labial productions in Lingula with those in Anomia, may be as true as it seems obvious; but in Arcaphala, developed on plans so different in the general disposition of the soft parts in the shell, the grounds for determining special homologies are by no means so satisfactory and convincing as in the different members of the animals organized on the same vertebrate type. The principle of a final purpose more obviously governs the number and position of the shell-muscles in Discina and Anomia, than any uniformity of plan to which such resemblance might be referred. But the dogmatic assertion of the abstract nature of the correspondences above cited, as being, e.g., of "analogy," not of "affinity," will not close the eyes of the impartial observer of nature to their presence and probable signification.

We have seen that the relation of the soft parts to the bivalve shell of the Brachiopoda is such that one valve is "ventral," the other "dorsal" in position. In the Lamellibranchiata one valve is applied to the right side, the other to the left side of the animal. The ciliated labial tentacles, as compared with those in the Brachiopoda, are short, and are more glandular than muscular; and there is usually a pair on each side of the mouth (fig. 11, h). Most of the Lamellibranchiata are free and locomotive. The instrument by which they move from place to place is a single symmetrical muscular organ (fig. 10, e) developed from the valve to valve is two. They are called "adductors," and Lamellibranchiata are antagonized by an elastic substance, so inserted at branchiata, the hinge uniting the two valves as that it is squeezed when the shell is closed, and opens the shell when the adductor muscles relax. The visceral mass occupies about half the cavity of the shell next the hinge (fig. 11); the rest of the interspace of the pallial lobes is chiefly occupied by the "foot," when this is developed, and by the "branchiae." The breathing organs (fig. 11, p, p) are mostly lamellose, or shaped like long subcrescentic leaves or plates, two on each side, dependent from the inner surface of the mantle-lobes (fig. 11, a). This characteristic of the breathing organs gives its name to the present class of Acephalous Mollusks. The heart consists of a ventricle with usually two auricles; and as a rule, the ventricle is traversed by the end of the intestine (fig. 17, q). An excretory organ, developed from the veins near the heart, communicates externally by the pericardial or aquiferous outlet. With the exceptions above mentioned the Lamellibranchiate Bivalves live with the back uppermost, resting on the ventral edge of their shell, as in the position of the sand-shell Pammoboa (fig. 10).

True Bivalves, as a general rule, are dioecious and ovoviviparous.

The well-known oysters, cockles, mussels, razor-shells, and ship-borers, exemplify the present class of Mollusca, of which all the members are aquatic, and most are marine.

The bivalve shell of the Lamellibranchs (fig. 12) offers, as might be expected, many modifications corresponding in general with those of the mantle, but otherwise related in a few species to boring habits and a peculiar locality; other calcareous parts in a tubular or other form being then usually superadded. The shell consists essentially of an organized extra-vascular combination of albuminous membrane and calcareous earth, chiefly carbonate of lime, arranged in successive layers. The innermost is the largest and latest formed, and each layer presents a cellular, lamellar, or prismatic texture, which presents characteristic variations in different families. Each valve of the normal shell is a cone, showing every grade of depth from the flat plate of the Placuna to the produced and spiral cavity of Isocardia and Diceras; it is commonly shallow, with the apex or umbo turned more or less to one side and directed forwards. If you place a bivalve shell in the position of the Cytheraea (fig. 13), the direction of the umbo determines A as the anterior border, and P as the posterior one; the upper or dorsal, and the lower or ventral borders, are as marked in the cut. The length of the shell is taken from A to P; its height or breadth at right angles from the dorsal to the ventral border; its thickness is measured across the closed valves, at the most prominent part from the right to the left side of the animal, as shown in fig. 12. Transfer yourself in imagination within the shell (fig. 13), with your head towards A and your back towards the dorsal border, and you will recognize the valve figured as the right valve. Anterior to the umbo there is usually an oval depression, forming a concavity in the outline.

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1 See Carpenter in Reports of the British Association for 1844 and 1847. Lamellibranchia of the valve; it is called the lamellibranchia. The hinge ligament is sometimes between the umboes, never anterior to them. If the shell be divided by a line dropping from the apex of the umbo into an "anterior" and "posterior" part, it is never equally divided; in other words, it is unequilateral. Pectunculus is least so; in Glycymeris and Solemya the anterior moiety is longer than the posterior one; in almost all other Bivalves it is shorter, as in Cytherea (fig. 13); commonly it is much shorter. Most Lamellibranchia are equilateral; that is, the right and left valves are of the same size and shape as in Cytherea (fig. 12). The exceptions occur in the stationary and often fixed species, which lie on one side when the lower valve is deeper and more capacious than the upper one. This lower valve in the oysters (Ostrea), in Pandora, and Lyonsia, is the left valve; the smaller and flatter upper valve is the right one. In Chamostrea and Corbula the left is the smallest valve. The Placuna, Pectines, Spondylus, and Arculidae rest on the right valve; the Anomiae are attached by degenerated muscular fibres passing through a hole or notch in that valve to a more or less calcified lamellar plug. All these shells are called inequilateral.

The bivalve is called close when the valves fit accurately; it is gaping if part of the borders do not come into contact when the shell is shut. In Gastrochaena this permanent opening is anterior, and serves for the passage of the foot. In Mya it is posterior, and serves for the passage of the byssus; in Solen and Glycymeris the shell gapes at both ends.

The outer surface of the shell is usually coated by an uncalcified layer of albumen called the periostracum or epidermis, continuous with the mantle. This surface is variously ornamented in many species, sometimes with ridges or "ribs," which may be either concentric and conformable with the layers of growth, or radiating from the umboes to the free margins of the valves; and the ribs may be bent, direct, oblique, or wavy. In Tellina fabula the sculpturing is confined to the right valve. In many species of Pholas, Teredo, and Cardium the surface is divided into two areas by a dorsal-ventral furrow or by a change in the direction of the ribs. The thorny oysters (Spondylus) are so called on account of the spines which project from the rib-lines; they are longest and in greatest number upon the non-adhering valve. In some conditions of the shell the spines expand into foliaceous forms.

The part where the two valves are joined together is called the hinge. The cardinal or hinge-line is short in Volutella, long and straight in Arcula and Arcas; of moderate length, and curved in most genera. The locomotive Bivalves have generally the strongest hinges; however, a very well-developed example of the hinge mechanism may be studied in the Spondylus. Certain projections or teeth of one valve interlock into cavities in the opposite valve.

The central teeth, usually beneath the umbo, are called Lamellibranchia cardinal teeth; those on each side are lateral teeth. The branchiata, Alasmodon and Kelia have only lateral teeth; in many Bivalves only cardinal teeth are present. The teeth are apt to become thickened and even obliterated by age, through irregular growth, or by the encroachment of the hinge-line. Some of the fixed and boring Bivalves are edentulous.

The soft mechanism of the hinge consists of the ligament and spring (fig. 16, T, b). The ligament is a tough, thickened portion of albuminous matter like that of the periostracum, and is usually attached to ridges on the outer (dorsal) part of the hinge-margin, behind the umboes; it is consequently stretched by the closing of the valves. The spring, sometimes called "internal ligament," and (though improperly) the "cartilage," is lodged in the furrows between the ligamental plates, or in pits along the hinge-line. It is composed of elastic fibres placed perpendicularly to the surfaces of attachment, so as to be compressed by the shutting of the shell, which the elastic fibres consequently tend to open as soon as the action of the adductors ceases. The two parts are very distinct in the genus of Bivalves, thence called Amphidesea (double ligament), but they co-exist in most genera, with alternate proportions; the ligament being small in Maestra, which has a large "spring," and large in Anodon, which has a small "spring." The Pholades have the spring, but have not the ligament. This is replaced by the homologue of the anterior adductor, which is so situated as to act as an opener of the shell, and is called the "umbonal muscle." The functions of the shell in this boring Bivalve are too active and too frequently in exercise to be performed by the passive antagonist of the muscular closing powers which suffices for ordinary Bivalves.

The formation and repair of the shell are due to the development, change of form, and calcification of cells from the mantle; the nucleated cells of its outer surface being the matrix of the nacreous layer, its thick and periodically glandular margin that of the opaque outer portion.

The microscopic structure of bivalve and univalve shells has been well illustrated by Professors Carpenter and Quekett.

The primitive nucleated condition of the cell is sometimes retained after calcification. The dissolved lime-salts, after endosmotic penetration of the organic walls of the modeling cell, obey so far the general crystallizing force as to polarize light. The forms of the constituent lime particles of the shell, so moulded by combined vital and polarizing forces, are manifold in the various genera of Bivalves. The shell of the Pinna, save a thin internal layer, is composed of vertical, slender, usually hexagonal prisms. A thin outer layer of the shell of the oyster also presents the prismatic cellular tissue; but in a great proportion of this shell nearly all trace of development from cells is lost. The gelatinous basis is lamelliform, and this variety is called the subnacreous shell-substance.

Fine tubes, analogous to those of dentine, permeate the thickness of this substance in many shells,—radiating vertically between the ribs in Area, vertical and scattered in the inner layer, and reticulate in the outer foliaceous spines, of the shell of Chama, which has an intermediate layer of ill-defined vertical prisms. The prismatic structure is rarely found, and then only in small proportion, in the Bivalves which have the mantle lobes in any degree united. The distinction between the internal or nacreous layers and the external or fibrous layer, has long been recognised, and has been forced, as it were, upon the notice of the palaeontologist by the circumstance of the two being often separated from each other in fossil shells, and sometimes from one having perished whilst the other remained. As the Lamellibranchiata, the two valves of the shell, and alone receives the impressions of the soft parts, the true characters of fossil shells, as those of the Arcuatidae and Radiolites, which, in consequence of their position in porous chalky beds, have lost all the nacreous layer, cease to be determinable, save when a natural mould of the interior has been formed before the pearly lining of the shell was dissolved. When the inner layer is preserved, its impressions reveal the organization of the ancient fabricator of the shell as clearly as do the forms and processes of fossil bones that of the extinct vertebrate animal. The layers of the thick subnacreous inner substance of the shell of the Spondylus have frequently wide interspaces, called, from their contents, "water chambers." This "camerated" structure is well shown in the right or lower valve of S. varius, fig. 19.

In fig. 14, showing the internal surface of a Cytheraea, the chief impressions are named. When two adductors are present, as in the Bivalves thence called "dimyary," they leave the anterior and posterior muscular impressions; when one adductor only exists, it answers to the posterior muscle, but is more central in position. The oyster is a familiar example of such "monomyary" Bivalve. When the pallial line or impression extends in an unbroken curve from the anterior to the posterior muscular impressions, it may be inferred that the inhabitant of the shell had either no siphon, or a very small or a non-retractile siphon; when the line is bent towards the centre before it reaches the posterior adductor, as in fig. 14, the presence of a retractile siphon is indicated, this notch being occupied by the retractile muscle of that part. When a foot is present, its retractor-muscles usually leave recognisable marks on the inner of each valve. The siphon in some of the elongated Inclua cannot be retracted into the shell; they are consequently exposed, as in Pholadomya and Pholas; such species derive extrinsic shelter by burrowing in the sand or stone. The Pholades have supplemental calcareous pieces in the hinge of the shell: two small plates protect the umbonal muscles, and a long narrow plate fills up the dorsal interspace of the valves. The Clavogella (fig. 15) and Aspergillum line their burrows with a calcareous layer d, which forms in the latter (fig. 23) a distinct tube, closed at the larger extremity by a perforated calcareous plate. One of the valves of the normal shell adheres to the tube in the Clavogella (fig. 15, f, g, h), and both are cemented to its inner surface in the Aspergillum (fig. 23). In the Teredo navalis (fig. 25) the valves are reduced to mere appendages of the foot at one extremity of the animal, and are almost restricted in their function to the action of boring. As the ship-worm advances in the wood, it lines its burrow with a thin layer of calcareous matter. The length of the body is chiefly due to the prolongation of each respiratory Lamellibranchiata, the siphon of which is provided with a small elongated branchiata, calcareous triangular paddle-shaped plate. In the Teredo gigantea the calcareous tube, which sometimes surpasses

6 feet in length, has parietes of from 4 to 6 lines in thickness, the texture of which is crystallized or spathose. Two tubes are developed within its siphonal end. In Teredo norvegica the calcareous tube is divided longitudinally, and also transversely, into compartments by irregularly-placed incomplete concave septa.

The valuable pearls of commerce are a more compact and finer kind of nacre, often developed in the substance of the mantle or around a particle of sand or other foreign body which has gained admission to the pallial cavity. The Meleagrina or Avicula margaritifera of the Persian and Indian seas is most famous for these productions; those developed in the gills or inner layer of the mantle are small and numerous; those of the outer layer are the largest but least regular, and are attached to the shell. The "external" pearls consist of a concentric layer of minute vertical prisms; the "internal" pearls of concentric layers of wavy calcified membrane.

The iridescent nacreous lining of the pearl oysters (Avicula), and many other shells, consists of the same wavy lamelliform tissue; the pearly lustre is due to the diffraction of the rays of light by the out-cropping edges of the lamellae, and in some cases to the minute plication of a single lamella. (Brewster.)

If the shell of a living pearl oyster be perforated, and a minute particle of sand introduced, it becomes a nucleus round which a pearl is developed. Linnaeus was knighted on making known to his sovereign this practical application of his science; but the artificial production of pearls had long been known to the Chinese, who obtain them of definite forms by introducing substances of the required shape into the shell.

The Unio margaritifera, or pearl mussel of British lakes and rivers, is fished up for the ornamental excretions to which it is subject. It is probable that the pearls from this source, collected by the ancient Britons, may have given rise to the statement of Tacitus, in his Life of Agricola, of "pearls not very orient, but pale and wan," being among the indigenous products of the conquered island.

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1 Annals and Mag. of Nat. History, vol. ii., 1838, p. 407. 2 Owen, Report on Animal Substances in the Great Exhibition of 1851, 8vo, p. 164. The mantle (fig. 16, ab) is that portion of the skin of the Lamellibranchia which, after investing the viscera, gills, and foot, is reflected, ventrad, in the form of plates and "lobes" to line the shell which it has formed, and be produced, when needful, into breathing-tubes. It consists of a middle layer of fibro-cellular tissue, in which contractile fibres are variously developed, of an internal layer of "ciliated epithelium," and of an external layer of epithelial cells, with nuclei, sometimes called "glandular epithelium."

The lobes of the mantle may be widely separated (Ostrea, Pecten (fig. 16, B, b); or partially united between the branchial and anal orifices (Mytilus); or united behind, leaving a single siphonal orifice; or united between the siphonal orifices and in front of the branchial opening; or the union of the mantle-lobes may be so extended as to leave a pedal or byssal opening, a branchial, and an anal opening,—all of small size (Dreissena); or finally, the mantle may become like a sheath, with terminal pedal and siphonal apertures (Solen); and the former may be reduced to a small foramen, and surrounded by a thick cushion of boring muscles, as in Clavogedda (fig. 20).

The mantle is complicated in some Bivalves by glandular sacs (fig. 16, a), which sometimes contain a gaseous fluid subserving the Pectens,—e.g., to float with their light shell, and change their situation at the ebb and flow of the tide. The Pectens, also, well exhibit the valvular reflected sub-marginal fold (fig. 16, h) of the mantle, the tentacula developed from that fold and from the free margin of the mantle (fig. 16, b), together with the eye-specks or ocelli e, near that margin.

The jawless and toothless mouth (figs. 16 and 17, C) is surrounded by a plicate (fig. 17) or tentaculate (fig. 16, F) labial border, produced into the labial palps (fig. 17, E), of very varying length in different Bivalves. The stomach (fig. 17, U) receives the hepatic ducts; and, save in most Monomyaria, develops a sac (fig. 17, V) containing the part called the "crystalline style," the gastric extremity of which (V) supports a thin cartilage, called the "tricuspid body" (fig. 17, v). The chief part of the style and of its sac is directed towards the pore by which water is admitted into the foot; and it has been conjectured to give some resiliency to that organ, with which it usually co-exists. In Anomia, however, it supports a free portion of the mantle. Poli supposed the tricuspid body to regulate the flow of bile into the stomach, for which its position seems to fit it. The intestine is short in Anomia and Ostrea, and exhibits various proportions up to twelve times the length of the entire Mollusk, as, e.g., in Cardium echinatum (fig. 17, D). Lamellibranchia. The curious relation of the rectum (P) to the heart Q is shown in this figure. The position of the rectum in the Pecten may be seen at D (fig. 16). The circulation may be traced, beginning with the abundance of veins which rise from the alimentary canal and from the superficial capillaries of the labial tentacles and generative mass, to the trunks receiving also the contents of the dilatable sinuses of the foot and peripheral part of the mantle, and pouring it into the median venous sinus, in which course it passes through the cavernous structure of the excretory (renal?) organ; the large sinus situated upon the posterior adductor and below the pericardium conducts the blood to the gills; but part of the veins of the mantle terminate, as Mr Garnier first observed, directly in the auricles. These, however, chiefly receive the blood from the branchial veins. Two semilunar valves regulate the flow of the blood from each auricle into the ventricle, and valves also prevent the reflux of the blood from the aorta into the ventricle. In Area, Lima, and some Pectunculi, the ventricle is divided. Whenever the veins expand into sinuses they preserve a proper tunic. In Ostrea and Anomia the ventricle lies upon the rectum.

The gills (fig. 11, p, p) are essentially internal highly vascular folds of the pallial membrane, and are strengthened by series of delicate-jointed filaments, which support several rows of curved vibratile cilia. The respiratory currents are occasioned by the ceaseless action of these cilia, aided by that of the ciliated epithelium of the mantle, and are not dependent upon any opening or closing of the valves of the shell. The ciliary action is that likewise which brings the nutrient molecules to the mouth, chiefly along the marginal grooves of the branchial plates, where the molecules are mixed with mucus, and moulded into small filamentary masses.

In Lucina and Corbix there is only a single gill on each side; as a general rule there are two. Each gill consists of two membranous plates, continued into each other at the free margin of the gill; the contiguous plates of the two gills are continuous at the base or bottom of the branchial interspace where they are fixed, the basal border of the plate forming the opposite side behind the foot in the fresh-water mussels. The two plates of each gill are united together at pretty regular intervals in the direction of their breadth by transverse septa or bars, so as to include canals running transversely to the gill-plates. According to the course of the current of water through these interlamellar canals, they commence by the small slits or pores along the groove on the free margin in the branchial chamber, and terminate by the wider openings at the fixed margin of the gill in the anal chamber.

The minute particles suspended in the branchial currents Lamellibranchiata are carried by the ciliary actions towards the mouth, and branchiata, the water is filtered through the interlaminar canals before it escapes. The walls of the interlaminar tubes support a regular network of blood-vessels, longitudinal and transverse,—the latter being most prominent; the meshes are parallelograms, and form open spaces fringed internally by a narrow ciliated membrane. The cilia compel the requisite movements of the water in the branchial chamber when the Bivalve remains suspended in the air, as happens to a mussel attached to a rock above low-water mark.

Even when the animal is in such a position as to be immersed only for about two hours in seventy-five days out of the year, it can live and grow, the retained water deriving oxygen from the atmosphere, and the animalcular food propagating therein. The life of the "tree oysters" (Ostrea polymorpha), suspended to the mangrove branches, is similarly explained through the wonderful mechanism of the ever-active microscopic cilia.

The two gills of one side are usually connected with those of the opposite side by their hind ends only, but sometimes the union is more extensive. In a few genera, as Anatina and Pholadomya, the two gills of the same side are so united as to appear like a single gill. In the Pholadomya this forms a thick oblong mass, finely plicated transversely, attenuated at both extremities, slightly bifid at the posterior one.

Although the microscopic cilia form the ordinary and constant dynamical part of respiration, the function is occasionally influenced by the muscles of the shell, as when the water is squirted out of the siphons by a sudden shutting of the valves. The quiet and ordinary respiratory current enters in Anomia at the anterior base of the shell, and escapes posteriorly near the termination of the branchiae. In Modiola vulgaris the current enters by the cirrigenous border of the mantle, and between that part and the foot; it escapes by the posterior produced part of the mantle. In Macra and Tellina, when the conjoined siphons are extended, and the hyaline valve is exerted from the anal siphon, the current flows in at the "inhalent" branchial or ventral siphon (fig. 10, g), and rushes out by the "exhalent" dorsal or anal siphon (fig. 10, g'); there is no current at the pedial aperture. The branchial siphon is often much dilated, so that its diameter greatly exceeds the anal one,—e.g., in Pholas. The siphonal apertures, especially the inhalent one, are provided with a circle of tentacles to prevent the ingress of noxious particles.

There is a remarkable plexus at the base of the gills, near the pericardium, which surrounds a distinct glandular organ in the higher Bivalves. It is double: each sac is elongated with glandular walls, and communicates with the pallial cavity by a small prominent orifice, usually close to the genital pores. The secretion of this body abounds with calcareous particles, and it was called by Poli the secreting organ of the shell. Modern analysis has detected uric acid in these sacs, and has thus determined them to be the renal organ.

An orifice at the extremity of the foot of Solen, at the middle of the foot of Cardium, and the tube situated above the pedunculate anus of Pinna, severally admit the sea-water into a reticulate system of channels in the substance of the foot, thence extending into the lobes of the mantles, and into a part of the visceral mass. By this provision for the admission of water the foot can be swollen out like a sponge, and made to exceed the capacity of the shell. The relations of this aquiferous system to the sanguiferous one are not satisfactorily determined. The fine jets of water expelled from the foot and the border of the mantle, when a Solen is suddenly removed from the water, are from the aquiferous canals.

The nervous system of the Lamellibranchiata is here illustrated by the modification which it presents in the common mussel (Mytilus edulis, fig. 18).

The labial or suboesophageal ganglia b may be distinguished by their yellow colour. At the base of the labial processes they are connected by a short transverse chord a, passing dorsad, and in advance of the mouth. Four sets of filaments diverge from the labial ganglia,—viz., the proper labial nerves, filaments to the hood-shaped process of the mantle above the mouth, nerves to the anterior adductor and continued into the circumpallial plexus g; and the principal nerve-trunks k, dividing or resolving on each side into the nerves to the pedial ganglion e, and those to the branchial ganglia x. The pedial ganglion completes the wide collar surrounding the cesophagus; it sends off nerves to the retractors of the foot, to the byssal ganglion, and to the acoustic sacs. The branchial or branchio-pallial ganglia x send off nerves which diverge as they pass to the base of their respective gills e; then each gives off a large nerve g', which passes across the post-adductor muscle to the hinder part of the pallial lobe, along which it curves, and is continued forwards near the border of the mantle until it meets and anastomoses with the corresponding nerve g, which was continued over the ant-adductor from the labial ganglion. These circumpallial nerves gg' send off branches which form loops at the base of the pallial tentacles and along the rest of the free border of the mantle, forming the circumpallial plexus h'h', which is continued into the cardinal plexus i; such continuation being unconformable with the circumpallial nerve gg'.

Oysters are sensible of light: it has been observed that they closed their valves when the shadow of an approaching boat was thrown forward, so as to cover them, before any undulation of the water could have reached them. The branchiata Pecten has a number of sub-pedunculate ocelli (fig. 16) arranged round the inner margin of the mantle, whence Poli called the animal Argus. But the name is equally merited by some other Bivalves. As many as sixty ocelli have been counted on the convex lobe of the mouth, and ninety on the plain lobe, in Spondylus geodromus.

The organ of hearing consists of a small sacculus, with thick transparent walls attached to the fore part of the pedial ganglion, and containing cretaceous sub-crystalline particles which perform oscillatory movements.

The labial tentacles seem well adapted, both by structure and position, in most Bivalves, to exercise the sense of smell; but in Nucula the palps are rigid. The mouth is highly susceptible of impressions from contact of its free, and commonly tentaculigerous border.

The nervous system in Acephalous Mollusks advances according to the complexity of their general organization, and especially of the muscular system.

To revert to the Tunicata, in a brief review of this progress and its chief physiological conditions, for which alone we have at present space:—The wants of the Ascidian are to breathe, to feed, to discharge its excrements, and emancipate its progeny. But the food is obtained by the same process as breathing, in this as in other Acephalous Mollusks; the streams of water which distribute oxygen over the surface of the vascular gill at the same time bring the nutrient particles to the mouth. This, moreover, begins at the esophagus. There is no true mouth, no jaws, no teeth; for there is nothing to comminute or prepare for deglutition. In ourselves, when at rest, respiration is an involuntary or reflex action. It is accelerated by volition, to meet, perhaps, the exigencies of some violent locomotive effort; but locomotion is denied to the Ascidian. The same stimulus of necessity which in man is carried, during sleep, to the medulla oblongata by the nerves vagus, and which is reflected along the phrenic nerves, causing diaphragmatic action, independently of sensation or volition, is, we may suppose, similarly impressed upon and reflected from some analogous nervous centre in the Ascidian.

Ordinary respiration in this animal is, however, effected by an organization of which the actions seem to be less dependent upon a nervous centre than those of any of the peristaltic moving parts; the respiratory medium is renewed upon the vascular surface of the gill by the action of the vibratile cilia. Occasionally, at intervals, the sac may be seen unusually distended, and the water to be expelled by a sudden and forcible contraction of the mantle,—an act which may be compared with the involuntary yawn by which we distend our lungs to the utmost, and empty them by a violent expiration.

In the Ascidian the contents of the rectum and the generative organs are expelled by the same action of the mantle, though by a different outlet from that which gives passage to and fro to the respiratory currents. Both outlets are provided with feelers and sphincters, to prevent the entry of noxious bodies, and to regulate the exit of the excretory-currents. The apparatus of nervous matter, in such a being, is a ganglion situated between the inhalent and exhalent orifices, as shown at fig. 5, k. Nervous filaments transmit to the ganglionic centre certain impressions; the requisite influence to cause contraction is reflected by other nerves to the muscular sac. But particles in the respiratory sac may be unfit for food, or hunger may require a greater or more frequent distension of that sac. Impressions are conveyed, therefore, from the mouth, or nutrient orifice, by nervous chords to the branchial ganglion; and, reciprocally, impressions received by the branchial ganglion are conveyed to the mouth.

In the oyster the mouth is released from the respiratory sac; it is so placed as to have a wider field of choice. We now therefore find it provided with "labial processes," or organs for selection, if not prehension: hence the necessity for their possessing nervous centres of their own. But the oyster becomes fixed, like the Ascidian; there is no locomotion in the adult state. Expulsion of respiratory streams, and of the excrements and sperm-cells or ova, by sudden approximation of the valves, and contraction of the pallial lobes, is the sole sign of vigorous motion, and it produces the same effect as the contraction of the muscular tunic in the Ascidian. The stimulus to such action is reflected from the ganglion upon the adductor to that muscle, and to the muscles of the pallial border. Such ganglion is, therefore, analogous to, if not homologous with, the ganglion k in the Ascidian: it is the "branchial" ganglion; the superadded nervous centre is the "labial" one.

In the mussel, with the additional muscularity and fringed structure of the mantle-lobes, and with that further development of the whole muscular system which governs an increased activity of the breathing function, the "branchial ganglia" are increased and parted. The foot, which, besides guiding and fixing the byssus-threads, is an exploratory organ, calls for an additional centre of the nervous system—the "pedial ganglion." Bivalves are conscious of light, but cannot discern an approaching object. The movements of the predatory crab or insidious whelk, whether by day or night, upon the rock from which the mussel swings, produce vibrations that will affect an acoustic organ. The tip of the protruded foot may be the first part to receive their appulse, direct or reflected, from the parts of the shell to which it is affixed. Certain it is, that with the pedial ganglion (fig. 18, r) is associated that little sac, with prismatic oscillating otolithes, which its discoverer Siebold and all subsequent physiologists hold to be a rudimental organ of hearing. The relations of this organ we shall find to change, in higher Mollusks, according as a ganglion may be situated near parts better adapted for the ready reception of the peculiar stimulus affecting it.

Thus the "pedial ganglion" is the third additional nervous centre in the Acephalous development. The fourth is where a small "siphonic ganglion" is developed at the point of confluence of the muscular respiratory tubes in the Bivalves which possess those accessory mechanical organs of respiration. The physiological explanation of this nervous centre is the same as that of the ganglion of the foot: the newly-developed parts extend and diversify the relations of the Bivalve with the surrounding world, and there must be corresponding centres for the reception and reflection of stimuli.

Take from man the necessity of clothing himself, by giving him a natural covering of scales or hairs, and you suppress, at the same time, all the arts which have attired for their object. Blunt his sense of taste, and the culinary arts must disappear; and so on for the other wants. Now, between the wants and the actions lie the faculties. The animals which have fewest organs have fewest wants, perform the fewest actions, and possess the fewest faculties. Hence the mystery which envelopes the psychical operations of animals is not so impenetrable as it seems.

The following table briefly expresses the principal characters of the soft parts upon which the Lamellibranchiata Bivalves have been divided and subdivided. It is based upon that given by Mr Garnett in his paper upon the anatomy of this class of Mollusks, with modifications suggested by subsequent observations of the writer and others—

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1 The chief steps of this progressive development of the nervous system may be found described in Owen's Lectures on Invertebrata, 1855, p. 510-511. 2 Zoological Transactions, vol. ii., p. 100. ### Classification of the Lamellibranchiata

**Labial tentacles very long, not distinct from the branchiae; branchiae united; adductors subdivided, and with an adductor-like byssal muscle.**

- **Anomia—Pleuura.** - **Ostrea—Gryphus.**

**Labial tentacles short, separate from the branchiae.**

- **foot long, cylindrical; ocelli at the edge of the mantle.** **Pecten—Pedum.** - **foot short, thick, with a disk at the extremity, from the centre of which depends a pedicellated oval body; ocelli.** **Spondylus—Plicatula.** - **foot compressed; no ocelli.** **Lima.**

**Mantle without separate orifices or tubes.**

- **foot slender, byssiferous; lips simple.** **Avicula.** - **foot long, rounded, deeply grooved, with a callosity.** **Arca.** - **foot compressed, crescent-shaped; its margin nodulated.** **Pectunculus.** - **foot oval below, its margin serrated; lips volute.** **Nucula.** - **foot large, pointed anteriorly, bent at an angle.** **Trigonia.** - **foot slender, byssiferous; anterior muscles small; byssus divided to its base.** **Mytilus.**

**Mantle with a distinct anal orifice.**

- **foot small, byssiferous.** **Modiola.** - **foot large, not byssiferous in the adult.** **Unio.** - **mantle widely open; foot long, sickle-shaped.** **Cardium.** - **mantle closed around the foot or byssus; foot small, cylindrical, bent at an angle; lips foliately.** **Lucina.**

**Mantle with an anterior and posterior orifice; not elongated into tubes.**

- **foot short and discal, byssiferous; anterior muscles small.** **Tridacna.** - **foot long, sharp; lips simple.** **Chama.** - **foot large, rather falceform; external branchiae shortened; mantle tentacular; labial tentacles small, pointed.** **Donax—Capra.** - **foot moderate, tongue-shaped; external branchiae shortened; edge of mantle simple; tentacles long, tapering.** **Psammobia—Soletellina, &c.** - **foot moderate; external branchiae rudimental; tentacles very large; margin of the mantle fringed.** **Tellina.** - **tubes small, partially divided; foot very long, obtuse.** **Cyclus—Cyrena, &c.** - **foot small, united to the extremity; foot large, beaded, and pointed.** **Macra.** - **foot larger; branchiae separate.** **Corbula.** - **foot not byssiferous; tubes large and coriaceous; lips long.** **Mya.** - **foot byssiferous; tubes moderate; lips short.** **Saxicava.**

**Branchiae not produced into the lower tube; mantle more or less open in front.**

- **foot long; club-shaped; tubes short.** **Solen—Sanguinolaris, &c.** - **two distinct adductor muscles; anterior one situated below a reflected portion of the mantle, filling the beaks instead of a cartilage; tentacles large.** **Pholas—Gastrochena, &c.** - **body very elongated; adductor muscles united; end of mantle with two calcareous pieces; tentacles small; no cartilage nor reflected portion of mantle.** **Teredo.**

**Mantle with two produced tubes, or siphons.**

- **foot very short, rounded.** **Clavagella—Aspergillum.**

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The Monomyaria, although they form an order or primary group of this class in most systems, are far from equivalent in the characters of their organization to the dimyaria group; and the same remark applies, though not with equal force, to the binary division based upon the absence or presence of siphons or respiratory tubes. The quinary division of the class, as proposed by Cuvier, expresses more natural and equivalent groups. The classification here... proposed is essentially that given in the writer's lectures on Lamellibranchiata. Invertebrata, 1855.

FAMILY I.—OSTREAD.R.

No anterior adductor; lobes of the mantle widely separated; foot none, or small and byssiferous. Shell inequivalve, sub-inequilateral; hinge, in some, edentulous.

Genera—Anomia, Ostrea, Placuna, Pecten (fig. 16), Lima, Spodolylus (fig. 19), Pedum, Plicatula.

Of this family, the oyster (Ostrea edulis)—the most valuable of British Bivalves—may be regarded as the type.

From the time of the discovery of Britain by the Romans the flavour of the oyster has been held in high esteem. The species is gregarious, and forms banks, usually on a rugged rocky bottom of from 5 to 20 fathoms depth, where they are obtained by the dredge. When collected with a view to the market, they are usually transferred to artificial grounds in shallow water, where they increase in size, but more tardily than in their native beds, not becoming full-grown until from five to seven years; neither do they breed so freely, and they are liable to fatal effects from severe frost. They, however, "fatten," as it is called, and acquire a more delicate flavour.

The "close" or spawning season is in May and June; the season for sale and distribution in London is from the 4th of August to the 12th of May inclusive. About 30,000 bushels of "natives," or oysters from artificial beds, and about 100,000 bushels of sea-oysters, are annually sent to the London market. The western coast of Scotland and the Hebrides offer conditions peculiarly favourable for the propagation and perfection of the edible oyster. In the artificial beds, which thrive best in sheltered bays and estuaries, the oysters sometimes acquire a green colour. On close examination, the colour is seen to be confined to particular parts. The branchiae and the intestinal canal beyond the stomach, e.g., are of a bright green; the liver is of a blackish-green instead of its usual reddish tint. The heart, nerve-ganglions, adductor-muscle, and mantle retain their normal colours. The above partial coloration is conjectured to be due to Confervae or green monads in the breeding tanks.

FAMILY II.—AVICULIDÆ.

A very small anterior adductor; mantle lobes separate; foot small, byssiferous.

Genera—Avicula, Meleagrina, Malleus, Vulsella, Gerillia, Perna, Crenula, Inoceramus, Pinna.

In this family the "hammer-oysters" (Malleus) are most remarkable for their form; the pearl-oysters (Avicula) and Pinnae most attractive on account of their products.

The Avicula with shells less oblique in form, and with flatter and more equal valves, showing the posterior pedal impression blended with that of the great adductor, constitute the subgenus Meleagrina, of which the proper "pearl-oyster" (Mel. margaritifera) is the most valuable species, on account of the nacre and pearls which it produces. The "nacre," or "mother-of-pearl," is the iridescent, pearl-like, inner layer of the shell. For this substance, which is used largely in ornamental manufactures, immense numbers of the "pearl-oyster" are annually imported: the "silver-lipped" variety from the Society Islands, the "black-lipped" from the Philippines, and a smaller kind from Lamelli-Panama. These fetch from L.2 to L.4 per ton. The branchiata nature of "pearl" has been already defined. The pure pearl of commerce is generally spherical, and has a yellowish-white or bluish colour, with a peculiar iridescent lustre. Its hardness is due to carbonate of lime, which dissolves with effervescence in acid, leaving fine films of membrane. Such pearls bore an almost fabulous price with the ancient Romans; their greatest naturalist testifies—"Principium culmenque omnium rerum pretii, margarite tenunt." (Plin. Hist. Nat., lib. ix., c. 35.) In modern times their value has declined. A handsome necklace of Ceylon pearls, in size less than a large pea, may fetch from L.170 to L.300; the single pearls selling at about a guinea each. Pearls of the size of peppercorns sell at from 2s. 6d. to 10s. Pearls of still smaller size are called "seed-pears." They are mostly sent to China. A pearl, pear-shaped, regular, and said to be without blemish, nearly 3 inches in length, and two-thirds of an inch in diameter, brought up at Catifa in Arabia, and now in possession of the Shah of Persia, has been valued at L.10,000. Mr Hope's pearl, exposed to view at the "Great Exhibition" of 1851, measures 2 inches in length, 4 inches in circumference, and weighs 1800 grains.

The best pearl-forming pearl-oysters are brought up from about 12 fathoms in the Persian Gulf, off the west coast of Ceylon, in the straits of Malacca, and at Tuticorin on the Coromandel coast. The largest pearls are found in the deepest places, as at the island Kharnak in the Persian Gulf. The Persian pearls are more esteemed than those of Ceylon. The pearl fishery of Tuticorin is monopolized by the East India Company, and that of Ceylon by the government; but it is very doubtful whether the results justify these exceptions to the general laws of political economy.

The animal of the Pinna, called Chimera by Poli, is remarkable for the abundance, length, fineness, and lustre of the filaments composing the "byssus." The part so called exists in many Bivalves; but in some it is peculiar to an early period of life; in none does it attain so great a development as in Pinna. The byssus-filaments are united towards their roots into a common mass; they are attached in a cavity at the base of the foot by a peculiar substance secreted there (conjunctive matter). From the cavity a furrow runs to nearly the extremity of the foot, and along this furrow is situated the glandular tissue by which the byssus is secreted. Réaumur maintained that these horny threads were spun just like the web of caterpillars and the nets of spiders. With the assistance of the muscular foot, these threads are directed to determinate situations, and caused to adhere there firmly by their broad disciform extremities; they grow again if cut off.

The byssus serves as an anchor to the Bivalve, of which use the common mussel offers a familiar instance. Vigorous strokes of the hatchet are needed to divide the byssus in detaching the great clam or "beniter" shell (Tridacna) from the rocks. The silky byssus-threads of the Pinna nobilis are woven into gloves and stockings in Sicily and Calabria; but they are objects rather of curiosity than of daily use.

FAMILY III.—MYTILIDÆ.

A small anterior adductor; mantle lobes united between the branchial and anal slits; foot cylindrical, grooved, byssiferous. Shell equivale; hinge edentulous.

Genera—Mytilus, Modiola, Lithodonmus, Crenella.

The common mussel (Mytilus edulis) is the most important member of the present family in regard to human food. Mussels are gregarious, in extensive beds, always uncovered at low water; they are as widely distributed as

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1 Lat. for "oyster." 2 Lat., signifying a "little bird." 3 A. Mueller de Bysso Aephalorum Acced., Tab. ii. Berolini, 1836, 4to; and in Wiegmann's Archiv. 1837, §§ 1–39, Tab. i., ii.

The oyster, and more easily attainable. The flesh is most esteemed and is least unwholesome in autumn; in spring, during the spawning season, cases of the gastric affection, followed by cutaneous eruption, called "musseling," are most common. They are, however, sufficiently exceptional to permit an immense consumption of this mollusk. It is estimated in Edinburgh and Leith at 400 bushels—say 400,000 mussels—annually; still greater numbers are collected for bait in the deep-sea fisheries. The species thus affords employment to numerous women and children of the fishing villages, especially along the Frith of Forth.

A singular form of fresh-water mussel (Mytilus polymorphus, Lam.), a native of the Arabo-Caspian rivers, is supposed to have been introduced with foreign timber into the Surrey Docks, where it was observed by Mr J. de C. Sowerby in 1824. It has since spread into canals, and has established itself as a British species. The anal and branchial slits are a little produced—the latter like a short siphon; and the rest of the mantle is closed, save at a small aperture through which the foot and byssus protrude. Accordingly, this mussel has been separated generically from Mytilus, as Dreissena, and under other names; and the Dreissena polymorpha departs widely from the pallial characters of the family Mytilidae.

In most dimyary Bivalves the foot is an organ of locomotion. To some which rise to the surface of the water it acts, by its expansion, as a float; to others it serves, by its bent form, as an instrument to drag them along the sands; to a third family it is a burrowing organ; to many it aids in the execution of short leaps. In the piddocks (Pholas), two strong muscles are sent from the foot to be attached to the rough spatulate terminations of the bent processes under the beaks of the valves; these serve as fulcra in the excavating actions of the foot.

We may generally observe, in relation with the greater development and more active function of the foot, a corresponding complexity of the respiratory system. This is effected by the superaddition of accessory organs in the form of tubular prolongations of certain parts of the margin of the mantle, which are provided with a special development of muscular fibres. These are called "siphons" (fig. 11, g, g').

FAMILY IV.—ARCADÆ.

Anterior and posterior adductors subequal; mantle-lobes separate; foot large, bent, deeply grooved. Shell equivalve; hinge long, multidenticulate.

Genera—Area, Cucullaea, Pectunculus, Limopsis, Nucula, Isoarea.

The animal of the Yoldia of Müller, otherwise nearly allied to Isoarea, departs from the pallial characters of the family in having the branchial and anal openings distinct, and produced into short retractile siphons. It makes a transition to the next family.

FAMILY V.—SOLENEILLIDÆ.

Mantle-lobes united behind with a single siphonal orifice or retractile siphon. Shell equivalve.

Genera—Solenella (foot deeply cleft, hinge with fine sharp teeth); Solemya (foot proboscisiform, hinge edentulous).

FAMILY VI.—TRIGONIDÆ.

Mantle-lobes separate; foot long, pointed, sharply bent. Shell equivalve, trigonal; hinge-teeth few, diverging.

Genera—Trigonia, Myophoria, Azinax, Lyrodema.

Of Trigonia only three living species or varieties are known; they are peculiar to Australia. Upwards of one hundred extinct species have been defined from fossil shells, ranging from the Trias to the Chalk, in Europe. It is remarkable that no species of Trigonia has been found in the Tertiary deposits of that continent.

FAMILY VII.—UNIONIDÆ.

Mantle-lobes united between the siphonal orifices and, rarely, in front of the branchial opening; foot very large, tongue-shaped, compressed, byssiferous in the fry. Shell usually equivalve; anterior teeth thick and striated; posterior ones laminar, sometimes wanting.

Genera—Unio, Castalia, Anodon, Iridina, Etheria, Mulleria.

The common fresh-water mussels (Anodon), and the river mussels (Unio), exemplify the present family. To the latter genus belongs the pearl-forming species (Unio pictorum, Lam.; Mya margaritifera, Linn.) of many of the rivers of Scotland. The family arrives at its maximum in regard to species and individuals in the large rivers and lakes of North America, and has furnished the subjects for the beautiful monographs by Mr Isaac Lea and Mr Kirtland in the Transactions of the American Philosophical Society, 2d series, vols. iii. and iv. (on the Naiads and species of Unio). In most of the Bivalves of the present family the fry are hatched in one of the gills, which swells out to such a size as to require a particular space in the shell for their lodgment; and the valves of the female are consequently more convex than those of the male.

The following dimyary families have short siphons and the pallial line simple:

FAMILY VIII.—CHAMIDÆ.

Pedal orifice small; foot very small. Shell inequivalve; hinge-teeth 2—1 (two in one valve, one in the other).

Genera—Chama, Monopleura, Diceras.

The species of the latter genus are extinct, and characteristic of the Middle Oolites. The shell is remarkable for the much-produced spiral umbones or beaks, which give the valves the appearance of rams' horns.

FAMILY IX.—HIPPURITIDÆ.

(Order Rudistes, Lam.)

All the genera are extinct. The best illustration of their nature and affinities is given by Mr Woodward.

FAMILY X.—TRIDACNIDÆ.

Pedal orifice large; siphonal orifices surrounded by a thickened pallial border, the anal one with a tubular valve; foot small, cylindrical, byssiferous. Shell equivalve, open; muscular impressions sub-central and blended.

Genera—Tridacna, Hippopus.

In this family the Lamellibranchiate type of organization is manifested under its largest size. The valves of Tridacna gigas are accordingly used to ornament gardens, as basins for fountains, and in Romanist churches for holding the holy water. The two large valves for this purpose in the church of St Sulpice at Paris measure each two feet in breadth. Linnaeus records a shell in the "Museum Ludovicus Ulrici" which weighed 532 pounds.

Hippopus maculatus, or the "bear's-paw clam" of dealers, is perhaps the most beautiful of Bivalves, whether in regard to form, texture, or colour.

FAMILY XI.—CARDIADÆ.

Pedal orifice large; siphons usually short; foot large, sickle-shaped. Shell equivalve, cordiform; cardinal teeth 2, lateral teeth 1—1, in each valve.

Genera—Cardium, Hemicardium, Lithocardium, Serripes, Adana, Conocardium.

The sapid Mollusk called cockle (Cardium edule) exemplifies the present family. It affects and often abounds in sandy bays, near low water. On the recess of the tide it burrows a few inches below the surface, its place being marked by a small depression. Women and children dig...

Family XII.—LUCINIDÆ.

Mantle usually widely open below, with one or two siphonal apertures; foot long, cylindrical or ligulate, sometimes byssiferous. Shell orbicular, closed; hinge-teeth 1 or 2, lateral teeth 1—1, or obsolete.

Genera—Lucina, Corbis, Ungulina, Kellia, Montecuta, Lepton.

Family XIII.—CYCLADIDÆ.

Mantle open in front; siphons more or less united; foot large, tongue-shaped. Shell sub-orbicular, closed; hinge with cardinal and lateral teeth.

Genera—Cyclas, Cyrena, Pisidium.

The species of this family live in fresh or brackish water. The fry of Cyclas are hatched in the internal gill.

Family XIV.—ASTARTIDÆ.

Mantle-lobes united behind by a curtain pierced with two siphonal orifices; foot thick or compressed, tongue-shaped. Shell equivale, closed; cardinal teeth 1—3, and usually a lateral tooth in each valve.

Genera—Astarte, Circe, Cyprina, Crassitella, Isocardia Cypria cardia, Cardita.

In the following families the siphons are long and retractile, leaving a sinuous "pallial line":

Family XV.—VENERIDÆ.

Mantle with a rather large anterior opening; siphons unequal, more or less united; foot tongue-shaped, compressed, sometimes grooved and byssiferous. Shell regular, closed; hinge with usually three diverging teeth in each valve.

Genera—Venus, Cythera, Artemis, Lucinopsis, Venerepis, Petricola, Glauconya.

"The shells of this tribe are remarkable for the elegance of their forms and colours; they are frequently ornamented with chevron-shaped lines. Their texture is very hard, all traces of structure being usually obliterated. The Veneridae appeared first in the Oolitic period, and have attained their greatest development at the present time. They are found in all seas, but most abundantly in the tropics." (Woodward).

Family XVI.—MACRIDÆ.

Mantle more or less open in front; siphons united with fringed orifices; foot compressed. Shell equivale, trigonal; hinge with two diverging cardinal teeth, and usually with lateral teeth.

Genera—Macro, Gnathodon, Lutraria, Anatinella.

Family XVII.—TELLINIDÆ.

Mantle widely open in front; foot tongue-shaped, compressed; siphons separate, long and slender. Shell usually equivale and closed; cardinal teeth not exceeding 2, lateral teeth 1—1, sometimes wanting.

Genera—Tellina, Pseudomria, Sanguinolaria, Syndosmya, Scrobicularia, Mesoderma, Donax.

In the following families the mantle is sheath-shaped, closed, save at the pedal and siphonal apertures. The group answers to the "Enfermes" or Inclusa of Cuvier. The species are burrowers in sand, mud, wood, or stone:

Family XVIII.—SOLENIDÆ.

Siphons short and united (in the long-shelled genera), long and partly separate (in the shorter and more compressed genera); gills prolonged into the branchial siphon; foot large, sub-cylindrical. Shell gaping at both ends.

Genera—Solen, Cultellus, Machara, Solecurtus.

The "razor-shells" or "spout-fish" (Solen silique and Lamellisolen curtus) exemplify the present family. They are branchiata, used in many places as food. Their burrows at low water are indicated by a small orifice like a key-hole, and they are found a foot or two beneath the surface. It is said that they never voluntarily quit their burrows; but fishermen in quest of them have the habit, in some localities, of throwing a pinch of salt in their holes, when they speedily rise to the surface, and are hooked out by a bent iron. If permitted, they soon bury themselves again. When used as bait for haddock or cod, the "razor-fishes" are kept for a day or two.

Family XIX.—MYACIDÆ.

Siphons united; foot small. Shell gaping behind.

Genera—Mya, Corbula, Neora, Thetis, Panopaca, Saxicola, Glycimeris.

The "gapers" (Mya arenaria and Mya truncata) exemplify the present family. They are found from a few inches to a foot below the surface, in mud or shingle. They are eaten in the Hebrides and in Zetland; and are collected for baits by fishermen.

Family XX.—ANATINIDÆ.

Siphons long, more or less united; foot very small. Shell often inequivalve.

Genera—Anatina, Pholadomya, Lyonsia, Pandora, Myochama, Chamostrea.

The "lantern-shells" (Anatina rostrata and other species) exemplify this family. Pholadomya has an external ligament only; the gills are compacted into a single mass on each side, and the mantle has a fourth opening for a supplemental rudiment of the foot.

Family XXI.—GASTROCHENIDÆ.

(Turbicolidae, Lam.)

Siphons long, united to near their free ends; foot finger-shaped, sometimes grooved and byssiferous; mantle with a boring disc in front. Shell regular, wedge-shaped, gaping in front; in some more or less cemented to a superadded calcareous tube.

Genera—Gastrochena, Claragella, Aspergillum.

"The Gastrochena modiolina perforates shells and limestones; its holes are regular, about 2 inches deep and ½ inch diameter; the external orifice is hour-glass shaped, and lined with a shelly layer, which projects slightly. When burrowing in oyster-shells it often passes quite through into the ground below, and then completes its abode by cementing such loose material as it finds into a flask-shaped case, having its neck fixed in the oyster-shell." (Woodward.) In the Gastrochena numia, Spengl. (Fistulana elva, Lam.), from the coast of Coromandel, the tube is free, and grains of sand often adhere to it; other species burrow in madreporites, and their calcareous breathing-tube is elongated to keep pace with the growth of the Polypes.

The "water-pot shells" (Aspergillum), and the "club-shells" (Claragella), inhabit each an elongated tube, the extremities of which correspond each to the anterior and posterior ends of the animal's body. The posterior end is widely open, expanding in a foliated form in the open sea; the opposite end communicates, by a varying number of minute and often-branched tubes, either with the interstices of a sandy bed, as in Aspergillum, or with cavities in the sandstone or coral rock in which Claragella is more commonly found imbedded.

From the greater resistance, therefore, which the nidus of Claragella presents to the development of the anterior tubes, these are less regular both in the number and place... than in *Aspergillum*; their formation being influenced by the proximity to the tube of other cavities in the rock.

Where these tubes have been developed from nearly the whole of the surface of the mantle which intervenes to the wide aperture of the shell, the specimen of *Clavagella* has been termed *echinata*, though these processes are not closed and pointed at the extremity, as might be implied by Lamarck's trivial denomination; where, on the contrary, their formation has been limited to the anterior part of the mantle, the specimen has been raised to the dignity of a species by the term *coronata*. *Clavagella*, like its congener *Gastrochaena*, is remarkable for the wide opening of the valves. These cannot in the natural state be brought into opposition on their ventral margins; they are unequal to the protection of the soft parts; and hence the necessity of the extraneous defence which the walls of their rocky chamber afford them, these walls being also lined to a greater or less extent with a calcareous exudation from the mantle. Yet a specific character has been seized from this disposition of the valves, indicated by the species called *Clavagella aperta*. The soft parts of *Clavagella*, when exposed by the removal of the free valve and outer layer of the mantle, present the appearance shown in fig. 20. The true foot is wholly concealed by the great development of the muscular margin of the mantle. The extremities of the labial tentacles protrude at the interval between the anterior adductor \( g \), and the retractor of the siphon. The relative position of the soft parts (fig. 20) to the dwelling-chamber (fig. 15) is as follows:

The mouth is turned towards the closed end of the chamber \( a \), which is the anterior part; the heart and rectum are nearest the side where the valves are connected by the ligament \( b \), or the dorsal part; the visceral mass projects towards the opposite or ventral side \( c \), while the siphon extends into the commencement of the calcareous tube \( d \), which leads out of the anal or posterior part of the chamber. The fixed valve (fig. 15) which covers the rough surface of the porous rock or coral like the tiling of a chamber floor, and affords a smooth, polished surface for the support and attachment of the animal, is the left valve; the right valve (figs. 21, 22) remains free, or is connected only to the soft parts and cardinal ligament, in order to assist in the excavating and respiratory actions.

That these actions are of a powerful kind is to be inferred from the remarkable development of the muscular system in the *Clavagella*. The impression of the great or posterior adductor (fig. 22, \( f \)) is carried two lines beneath the surface of the chamber posteriorly, but gradually rises to the level of the valve. The impression of the smaller anterior adductor (fig. 22, \( g \)) is fainter, and is continued into the sinusoidal pallial impression which follows the contour of the anterior margin of the valve at about two lines' distance from it. In the free valve (fig. 22) the last two muscular impressions are separate.

The shelly substance of the fixed valve passes without interruption into that of the tube; a slight ridge circumscribing the entry of the tube into the chamber may be regarded as the line of separation, unless the extent of the valve be limited to that of the internal nacreous deposition.

The area of the tube is of an oval form. The calcareous lamelliparietes are 1-50th of an inch in thickness at the outlet of branchiata, the tube, and about 1-30th at the opposite extremity.

The free valve (figs. 21 and 22) is an unequal triangle, with the angles rounded off, about the thickness of a sixpence, moderately concave towards the soft parts, striated only in the direction of the layers of increment on the outer surface. The layers of increment of the free valve gradually increase towards the dorsal edge for a little more than one-half of the valve, beyond which the layers continue of almost equal breadth. Free valve (*Clavagella aperta*).

This growth of the valve corresponds to the direction in which the chamber is enlarged, which is principally on the dorsal, dextral, and anterior sides. Now this is the mode of enlargement best adapted for the full development of the ovary; so that it would seem that the *Clavagella* continues for a certain time to work its way into the rock without material increase of size, leaving behind it a calcareous tube which marks its track, after which it becomes stationary, and limits its operations to enlarging its chamber to the extent necessary for the accomplishment of the great object of its existence.

The mantle envelopes the body like a shut sac, but is perforated for the siphon and foot, the opening for the latter being reduced to a small slit marked by the insertion of a bristle in fig. 20, which serves to keep up a communication between the chamber and its inhabitant; and it is seen that the chamber has always a communication with neighbouring cavities in the rock by means of the calcareous tubules, the formation of which is determined by the proximity of those cavities. When therefore the *Clavagella*, by a sudden contraction of its adductor muscles (fig. 20, \( f, g \)), has forcibly expelled the branchial currents from the siphon, the space between the free valve and the walls of the chamber would be simultaneously filled either by water rushing in through the tubules or forced out from the branchial cavity through the small pedal orifice of the mantle.

The outer dermoid layer of the mantle is extremely thin, and where it does not line the valves, it is mottled with minute dark spots, and presents a glandular appearance under the microscope. The muscular layer, after forming the siphon and its retractors, is confined to the anterior part of the mantle, where it swells into a thick convex mass of interlaced and chiefly transverse fibres attached to the valves along the sinuous submarginal depression above mentioned, and forming one of the principal instruments in the work of excavation. No fibres could be detected in other parts of the mantle, nor could any longitudinally radiating muscles be expected in a mantle which had no lobes to be retracted.

The siphon, in the contracted state which it presented in the specimen dissected by the writer, formed a slightly compressed cylindrical tube half an inch in length, and the same in the long diameter. It is traversed longitudinally by the branchial and anal canals, which are separated from each other by a muscular septum extending to the end of the siphon, beyond which the two tubes do not separately extend outwards. The inner extremity both of the anal and respiratory tube is provided with a valvular fold. Their terminations are beset with short papillae. The retractor muscles attach the siphon to the posterior adductor on one side, and to the anterior extremity of the oval mass of muscular fibres above mentioned on the other, leaving an intermediate space on both sides of the body, which exposes part of the gills and labial tentacles. The muscular mass which bounds the anterior part of the animal's body is of an oval form, 1 inch 3 lines in length, 8 lines in breadth, Lamellibranchiata, and varying in thickness from 2 to 3 lines. It is smooth externally, and convex internally, and hollowed out within to lodge the viscera at the base of the foot, for the passage of which it leaves the small orifice above mentioned. The margins attached to the valves are more or less irregular; that which is affixed to the loose valve is the broadest, being at the ventral extremity 3 lines in breadth; it may here be regarded as a third adductor. Posteriorly it is continued into the small adductor muscle. This muscle is marked \( g \), the great or posterior adductor \( f \), in fig. 20.

The ovary is of a gray colour, forming a mass at the dorsal aspect of the body above the great adductor muscle, and extending ventrad on either side the esophagus and stomach to the opposite end of the base of the foot.

All this mass of intestinal folds, hepatic follicles, and ova, was covered by a thin membrane. The little muscular process or foot which passes through the anterior slit of the mantle is but 4 lines long and half a line in breadth.

The organization of Clavogella, like that of Aspergillum, described in the Reise von Afrik of Dr. Rüppell, is thus seen to be modelled on the Lamellibranchiate type, and follows most closely, in the variations from that type, the modifications which have been observed in Gastrochena.

The lengthened worm-like figure of Aspergillum is exchanged in Clavogella for a shorter form, with greater lateral development; and instead of the small rudimentary valves, which are inclosed, as it were, in the calcareous sheath of Aspergillum, we find them here largely developed, and one of them always remaining at liberty to be applied by a powerful muscular apparatus to those offices which are essential to the forcible expulsion of the fluid in the branchial cavity.

The Aspergillum exhibits the most extreme modification of the true Bivalve type of shell by the complete coalescence of its valves (fig. 23, \( a \)) with the parietes of the shelly tubes. They there form the stamp of its true affinities, but sub-

serve as little any ordinary final purpose as the teeth buried in the gums of the foetal whale.

The larger end of the tube is pierced by a number of short and small tubules and by a minute central fissure; the opposite or siphonal end is open, it is either plain or ornamented by circular plicated outstanding plait formed by the shell-secreting layer of the siphons, and indicative of successive phases of growth of the tube.

The Aspergillum vaginiferum (fig. 23) inhabits the Red Sea; other species have been found in Java, Australia, and New Zealand.

**Family XXII.—Pholadidae.**

Animal clavate or vermiform; siphons large, long, united nearly to their ends; foot short, truncate. Shell gaping at both ends; hingeless, with sometimes accessory valves, or a supplementary tube and palettes.

Genera—Pholas, Xylophaga, Teredo, Teredina.

The piddocks (Pholas) perforate all substances that are softer than their own valves. The mantle extends over the hinge, and, in Pholas dactylus, develops two accessory plates to protect the umbonal muscle, and a small transverse plate behind; a long unsymmetrical plate fills up the dorsal interspace of the valves behind the hinge. The prominent parts of the valves are beset with calcareous inequalities, connected by fine transverse parallel ridges, roughening the outer surface like a rasp. This species is used for food in Scandinavia, and for bait in the south of England; the hyaline style, lodged in the translucent foot, is worthy of note in the recent animal. The common piddock is gregarious. Fig. 24 shows a block of stone perforated by the

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Fig. 23. Aspergillum vaginiferum.

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Fig. 24. Pholas dactylus.

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Pholas dactylus: the breakwater at Plymouth is thus infested.

In Pholas dactylus the labial processes are relatively longer than in Pholadomya, but are equal, and both have the usual transversely-plicated structure, although the outer one adheres by a great part of its outer surface to the mantle. In the Pholas crispata the whole external surface of the labial process is adherent, but it retains the fine plicated membranous structure; the internal labial process is a thick, broad, scarcely flexible mass, convex outwardly, concave where it is closely applied to the visceral mass, of a soft parenchymatous structure, quite smooth on the con-

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2 Gr. verb, "to be hid in a hole." cave side, with hardly perceptible traces of the transverse branchiata, lineations on the convex surface. This condition of the inner labial process has not been found in other Bivalves.

The ship-worms or pile-worms (Teredo navalis) live in wood, which they perforate in the direction of the grain, save where a knot in the timber or the tube of another Teredo is met with. The cavity in which they reside is lined with a calcareous incrustation. They grow in the wood, and do not first enter it as adult animals, for the external aperture, towards which the two tubes (palettes) are turned, is too narrow to allow the inclosed animal to enter. In fig. 25 the small valves of the true Bivalve shell are shown at \(a\); the foot, which with the valves constitutes


the boring organ, is shown at \(e\); \(d\) is the liver and alimentary canal; \(c\) the branchiae; the siphon-tubes and "palettes" terminate the vermiform body, and always point to the outlet of the burrow.

For the distinction of species recourse is had, amongst other characters, to the form of the two small shovel-shaped calcareous plates at the base of the siphons, commonly named palettes or palmulae. One species, famous for the injury it caused to the piles of the dykes in Holland, in the beginning of the last century especially, is referred by some Dutch naturalists to the Teredo Sellii (Teredo navalis, L. et auctor. in part, Teredo batarus, Spengler). It is figured in the work of Sellius, and in Blumenbach, Abb. naturhistorischer Gegenstände, No. 89. The palettes are fixed on a short pedicle, inversely triangular, and terminating at the broad end on each side in a point. Piles that had been driven only six or seven weeks previously were seen to be entirely eaten through by this worm, and robbed of all their strength. In this way the island of Walcheren was in 1730 threatened with destruction. From time to time the same mischief was discovered in other places, especially on the Zuiderzee near Medemblik, Lambertshagen, &c.; West Friesland was forced, in consequence, to mask its dykes with large stones, which, being brought into the country from abroad, occasioned a great expense. Since the middle of the last century the mischief has much diminished. Copper-sheathing and broad-headed iron nails have been found most effectual in protecting ship-timbers and piers from the ravages of this singular little Bivalve. An excellent account of the pile-worm is given in the work of G. Sellius, Histor. nat. Teredinis, Traj. ad Rhen. 4to. Fossil Teredines are found in the Tertiary and Chalk strata.

The phenomena of boring substances of different kinds and densities by the Bivalve Mollusks have been the subject of many observations and experiments, and have suggested many hypotheses.

The peculiar shape and development of the foot in the Solen and other "burrowing" Bivalves, might have led to its recognition as an excavating agent, if even it had not been seen to effect the purpose in the living Mollusk. Direct observation of the "boring" Bivalves in the act of perforation has been rarely enjoyed, and the instruments have been more frequently guessed at or judged from the structure of the animal. The peculiar shape, great strength, and restricted size of the concentrically-ridged valves, the disproportionate size and strength, and the red

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1 In the great Teredo arenaria, which lives in soft mud, the valves are wanting, according to Dr Gray; or their homologues form the convex cap closing the periodical growths of the calcareous tube.

2 On the Barrowing and Boring Marine Animals, Philosophical Transactions, 1826.

3 On the Anatomy of the Lamellibranchiate Bivalves, Zoological Transactions, vol. ii., 1839.

4 All these rare species were liberally confided for examination to the writer by Mr H. Cuming. face of the cushion, which was perforated by a minute slit for the occasional passage of a filamentary foot. In the original account of its anatomy it was thereupon suggested that this muscular development of the mantle must be "one of the principal instruments in the work of excavation." But viewing its attachment to the moveable valve, and the strength of the adductor muscles, it was also supposed that that valve might be applied not only to effect the forcible expulsion of the fluid from the pallial cavity, but probably to assist in the excavation of the abode." Mr Hancock has recalled attention to the excavating agency of soft and muscular masses in other boring Bivalves analogous to that in Clavogella; such as e.g., the thickened portion of the mantle in Saxicava and Gastrochena, and the foot in Pholas and Teredo.

If siliceous particles be actually secreted in the surfaces of any of the burrowing discs, they must add to their efficiency; and it is certain that the perpetual renewal of a softer surface will render it capable of wearing away a harder one, subject to the friction of such softer surface, and not, like it, susceptible of being renewed.

The admission of the wearing and boring power of muscular discs need not, however, involve the rejection of the allied action of shelly valves and ciliary currents. The diversity of the organization of the boring Mollusks plainly speaks against any one single and uniform boring agent in all.

The action of the foot and thickened border of the pedial aperture may be inferior to that of the valves in Teredo, as it certainly is in Pholas. A valued correspondent, Mr Robertson of Brighton, informs the writer— "Between thirty and forty Pholades have been at work in lumps of chalk, in a finger-glass, and a pan of sea-water, at my window for the last three months. The Pholas dactylus makes its hole by grating the chalk with its rasp-like valves, licking it up, when pulverized, with its foot, forcing it through its principal siphon, and squirting it out in oblong nodules. They turn from side to side, never going more than half round in their hole, and cease to work as soon as the hole is deep enough to shelter them." The Pholades attain their largest size in soft yielding stone; whilst in hard, and especially gritty rocks, they are dwarfed in size, and the rough surface of their shell is worn away. M. Caillaud has shown that the "valves are quite equal to the work of boring in limestone, by imitating the natural conditions as nearly as possible, and making such a hole with them."

The foot of the Pholas affords the requisite external fixed point or fulcrum on which the power of rotation by internal muscular motion depends. In Lithodomus (the Dactylus or date-shell of the conchologists prior to Linnaeus), in Saxicava and Ungulina, the foot is too feeble to serve as such a fulcrum, and the valves are smooth, and retain the periostracum; yet they bore into the hardest marble, and still harder shells; their holes, like that of Clavogella, are not cylindrical, and are doubtless formed, as in that genus, by the agency of the thickened muscular borders of the pedial aperture.

Teredo navalis bores in the direction of the grain, unless it meets another Teredo, or a knot in the timber; they are probably warned by their organ of hearing of such contiguity. The rasp dust is introduced by the foot into the pallial cavity, and is swallowed. The long intestine of the ship-worm is usually laden with this debris.

**GENERATION AND DEVELOPMENT OF THE LAMELLIBRANCHIATA.**

All Bivalves are richly prolific. Countless myriads of sperm-cells are developed in the one sex; and the ovarium swells with germ-cells as countless in the other sex. When the sperm-cells in the male oyster, e.g., have been developed branchiata, into moving filaments, and are excluded as such, these are drawn into the pallial cavity of a contiguous female along with the respiratory currents of sea-water; and at this season the oviducts are seen to contain a milky fluid abounding with spermatic filaments. By virtue of the action of the vibratile cilia of the mantle and gills, impregnation is effected in the Bivalves of distinct sex; even when, like the oyster, both male and female are cemented to the rock; just as the pollen of the rooted male of the dioecious palm is wafted by currents of air to the moist stigma of the equally fixed and rooted female tree.

In the Naiadæ, as has been remarked, the impregnated ova are hatched in the cavity of the outer gill, as in a marsupial pouch. The germ-mass, in the progress of development, becomes covered by a ciliated epithelium. In the group of four eggs of the Unio litoralis (fig. 26), the little arrow indicates the course of the rotation of such advanced germ-mass in the fluid albumen which divides it from the chorion, or transparent flexible "egg-shell."

This singular phenomenon was discovered by Leeuwenhoek in 1695. When the rotation of the embryo is most active, seven or eight revolutions may be observed in one minute. Two parallel fissures next divide the germ-mass, at the bottom of which the visceral mass first appears. As it protrudes, the diverging moieties of the germ-mass assume the character of the "mantle-lobes," or rather of a body potentially including the gills and shell with those lobes, those three parts being subsequently differentiated. The gills make their appearance as ciliated, wavy folds (fig. 27, b) from the inner surface of the mantle, near the angle between the pallial lobes and visceral mass. The cleavage of the primitive germ-mass appears at first like an attempt at spontaneous fission. Each embryonal moiety has its own mouth, stomach, and heart; and it is by the subsequent approximation and fusion of the two ventricles that the common rectum of the two originally distinct intestines is intercepted, so as to seem to pass through the heart. The rest of the alimentary canal blends with its fellow as the visceral mass grows up from the bottom of the cleft, which growth is due chiefly to the progressive increase of the testis or ovarium; and thus two sexless individuals combine to form one with sexual organs.

By the phenomena observed in the progressive building up of the embryo Lamellibranch, we learn the cause or chief condition of that most singular anatomical fact, which previously was a mere empirical one, without intelligible explanation—viz., the passage of the rectum through the centre of the ventricle of the heart. Calcification commences at the outer surface of the lobes, and the first layer of the future shell forms a small triangular valve on each side. The development of the adductor muscle, single at the beginning and near the hinge, is indicated by feeble attempts at opening and closing the valves. The albumen during this development is absorbed and assimilated, and the embryo now distends the chorion. The large detached ovum in fig. 26 exhibits the embryo Unio, or fresh-water mussel, ripe for exclusion. It escapes from the chorion before it quits the branchial marsupium. Filamentary processes, twisted together, resembling a "byssus," project from the visceral mass. The borders of the valves are broken and armed with spines. The young Encephala of all the Naiades anchor themselves, soon after quitting the parent, by this byssus, which is temporary; the full-grown and strong-shelled animals do not need it.

The embryos of many Bivalves have locomotive structures equally temporary. Those of *Crenella marmorata*, e.g. (fig. 27), show a broad disc fringed with long cilia \(d\), and provided with a slender tentacle \(f\), which seems to be an organ of exploration. As the pallial lobes and valves \(e\), \(v\) grow, the swimming disc decreases, and gives place to the labial palpi and the foot. The youthful excursion being ended, a byssus is finally formed to anchor the maturing Crenella to its final place of settlement.

In fig. 28, the embryo of the mussel (*Mytilus edulis*, after Lovén), \(e\) marks the eye-speck which disappears with the locomotive power; \(e'\) is the acoustic vesicle; \(h\) the labial tentacles; \(ss\) the stomach; \(l\) the liver; \(b\) the branchiae; \(h\) the heart; \(v\) the vent; \(a\), \(a\) the anterior and posterior adductors; \(f\) the foot.

**MOLLUSCA ENCEPHALA.**

About three-fourths of the Mollusca are "Encephalons," or have a distinct head, commonly with eyes and tentacles, and the mouth has a peculiar and complex preparatory organ of digestion.

The mantle, properly so called, is to be distinguished, as in the foregoing group, from the ordinary external tegument. It is the free fold or folds of the skin, produced usually from the dorsal surface, and is in functional relation with the breathing organ and the shell.

Any part of the skin of a Mollusk, even that covering the foot, as in *Lithedolus*, may produce a calcareous plate, but it is, as a rule, the function of the mantle-lobe or lobes.

In the following descriptions of the Cephalous Mollusks, the part answering to that marked \(aa\) (in fig. 70), is called the anterior or fore-part; the opposite end is the posterior or back part. \(B\) denotes the ventral or lower surface, \(C\) the dorsal or upper surface of the body.

The parts called "horns" or tentacles, but which are organs of sense or of exploration, not of prehension or offence, are in two pairs in many Encephala. The anterior pair, in which the olfactory sense may well reside, are usually also less dorsal in position than the posterior pair, with which the eyes are frequently connected.

The acoustic sacs are connected with the suboesophageal or pedial ganglia in most Encephala, but receive their nerves from the supersessophageal ganglia, in some species. In Encephalous Mollusks, as in fishes, the auditory concretions present themselves in one of two forms,—viz., as solitary otolites or as granular otocones.

A very characteristic feature of the organization of the head in the Cephalous Mollusks is the complex dentigerous rasping or boring organ (figs. 29 and 96), called by some the "buccal mass," by others the "proboscis," and by others the "tongue." As it coexists with lateral horny jaws in some species, e.g., *Eolis* (fig. 29, c) and *Tethys*, and with vertical ones in others (*Cephalopods*, fig. 96), it seems most homologous with the protractile dentigerous tongue in Myxinoïd and other fishes; but as precise homology cannot be predicated of the parts of animals developed on plans so diverse as the vertebrate and molluscous, it seems best to designate the apparatus in question by an arbitrary "molluscous" term, e.g., "glottidium."

This is formed by a basis ("chondrium") \(d\), of usually cartilaginous consistency, composed sometimes of a single piece, as in the *Aplysia*, but more usually of a pair, or with accessory pieces; the limpet, e.g., has four "chondria." The chondrium is convex towards the upper and fore part of the mouth. It is more or less enveloped by muscular fibres; some (*glottidiales*), \(e\), passing to the cephalic parietes for the protrusion and retraction of the entire glottidium; others (*radulares*), \(f\), inserted into the "radula" \(g\). The rasping part, so called, consists usually of an elastic membrane stretched over the convexity of the chondrium, and supporting a spinous membrane or "tooth-strap."

The "glottidiales" are protrusors and retractors. The protrusor muscles are usually two on each side, which pass forwards to be attached to the walls of the head or to a more advanced part of the mouth-channel. The "radulares" are usually in two sets, radiating and transverse. The radiating series, \(f\), \(f\) are the thickest, and often present a reddish colour. They arise from the back and under part of the "chondrium," and diverge as they ascend to be inserted into the extremities and sides of the radula. The thinner transverse fibres, when present, combine with the more constant diverging ones, in moving the radula to-and-fro upon the chondrium alternately raising and depressing the denticles by the same rasping or sawing movement.

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1 Gr. γλῶττα, "tongue." 2 χονδρός, deriv. from χονδρός, "gristle." 3 Lat. for "rasp." This mechanism seems to have been much misunderstood. Mr Huxley, for example, states that "the resulting action is precisely that of a circular saw;" but the distinctive action of the circular saw is to rotate in one direction; the "radula" of the Mollusk acts like a common saw or rasp, by alternate movements to and fro.

The lining membrane of the mouth commonly protrudes in folds near the glottidium, as at h (fig. 29) in Eolis and in Onychotenthus (fig. 96). The epithelial covering of the inner side of these folds in the Cephalopod is beset with series of short retractile horny teeth. Another complexity in the higher Mollusks, and one which adds to the lingual analogy of the glottidium, is the development in advance of the "radula" of one or more soft papillose and apparently gustatory lobes i. These caruncles have special retractor muscles; and the anterior "radulares" derive in Cephalopods an origin from the base of the lower jaw.

In fig. 29 of the mouth and glottidium of a Nudibranchiate Mollusk, a is the outer lip; b the inner lip; c is the margin of the vertical jaw; e the fleshy mass supporting the jaws; e the constricting muscle of the mouth; d the "chondrium;" e the "glottidiales" muscles; f the "radulares;" g "the tongue-strap;" h the buccal fold; I the oesophagus.

The teeth are set upon either a broad or a narrow band; in the former a median series of teeth or "rachis" is flanked by two lateral series or "pleura," as in the Gastropod called Bezoardica glauca (fig. 30). In some the "rachidian" teeth, in others the "pleural" teeth, are absent; and the number of longitudinal rows of these divisions may also differ in different genera. According to these differences, which, from their constancy, are valuable as generic characters, they may be indicated by the terms uniserial, biserial, triserial, septiserial, or multiserial, as, e.g., in Melania the rachis is uniserial, the pleura triserial. Whether the "chondria" or lingual cartilages consist of a single piece thinner in the middle line, or of two or four pieces united by ligament and muscle, the dental sac is lodged at the back, and the dental band is stretched over the upper and fore part of the chondrium. Herr Troeschel has devoted twenty years to the study of the dentition of the Mollusks, which he regards as important as that of any vertebrate class of animals.

The peculiarities of the circulation in the Encephalous Mollusks, which have given rise to the erroneous ideas of the absence of proper walls to the circulating sinuses, and the conditions originally attributed to that system in certain Encephala, under the term "phlebenterism," make it desirable to premise a few observations on the true state of the diffused venous system in other classes of Invertebrata.

John Hunter first observed, that "the veins of the insect would appear to be simply the cellular membrane; but they are regularly formed canals, although not so distinctly cylindrical canals as in the quadraped, &c., nor branching with that regularity. They would appear to be, or to fill up, the interstices of the flakes of fat, air-cells, muscles, &c., and therefore might be called, in some measure, the cellular membrane of the parts." (Hunterian MS. Catalogue, printed in the Physiological Catalogue, tom. ii., p. 31 (1834).

Baron Cuvier, as is well known, entertained, with regard to the vascular system of insects, ideas closely akin to those which some of his pupils have more recently expressed by Encephala, the term "phlebenterism;" for Cuvier supposed that the whole of the blood of insects stagnated in the lacunae or cellular interspaces of the several organs; he was consequently led to deny that insects possessed a true circulation, or that the dorsal tube ("heart, extending through the whole length of the animal," Hunter, op. cit., 1793, p. 137) acted as a heart. The more truthful views of Hunter, based on the analogy of the already commencing irregularity and extent of the venous sinuses in the lobster and snail, have been amply confirmed by the researches of Professor Carus, on the "Circulation of the Blood in the larva of Ephemerides and Libellulae."

With regard to the Crustacea, Hunter, who left preparations, and a beautiful series of drawings, illustrative of the circulating system in the lobster (Astacus marinus), thus describes the latter:

"The veins in this class of animals, as in the winged insect, &c., are principally in the form of large irregular cells, as if the cellular or investing membrane of the animal contained the venal blood; and, when injected, we find the injection principally in large masses." He then, referring to his figures, describes the different sinuses, as "a, a large mass of vein lying on the stomach; b, another mass similar to the above, lying principally on the heart, which might almost be considered an auricle, as from it are openings into the ventricle." (Hunterian MSS., in Physiological Catalogue of the Museum of the Royal College of Surgeons, vol. ii., 1834, p. 138.)

The conditions of the vascular system in insects, crustaceans, and the snail, enunciated in brief but clear general terms in the work On the Blood, 4to, 1793, and exemplified by preparations, drawings, and manuscript descriptions left by Hunter in his museum, at his demise in the same year, appear to have passed unappreciated abroad, until the successive discoveries of analogous structures in other invertebrata, or the re-discovery of the same structures in the same species which Hunter had dissected, had been made.

Baron Cuvier appears to have been the first to recall the attention of comparative anatomists to this diffused and expanded condition of the venous system in his dissection of the Aplysia; but he mistook the expanded sinuses which fill the abdominal cavity, like those in the snail and slug, for that cavity itself. Describing the vene carce, which perform the office of the branchial arteries, he writes:—"Leurs parois se trouvent formées de rubans musculaires transverses et obliques, qui se croisent en toutes sortes de sens, mais qui laissent entre eux des ouvertures sensible à l'œil, et encore plus à toutes les espèces d'injections, et qui établissent une communication libre entre ces vaisseaux et la cavité de l'abdomen; de manière que les fluides contenues dans celui-ci pénètrent aisément dans ceux-la, et réciproquement."

"Il résulte toujours que les fluides épandus dans la cavité abdominale peuvent se mêler directement dans la masse du sang et être portés aux branchées et que les veines font l'office des vaisseaux absorbants. Cette vaste communication est sans doute un premier acheminement à celle bien plus vaste encore que la nature a établie dans les insectes, où il n'y a pas même de vaisseaux particuliers pour le fluide nourricier."

Jurine, in 1806, observing living specimens of a minute crustacean (Argulus foliaceus) which were asphyxiated by a few drops of alcohol added to the water they were in, traced the course of the circulation, which he describes, remarking:—"J'ai évité d'employer le mot vaisseau pour désigner les conduits dans lesquels le sang circule, et que j'ai remplacé ce mot, tantôt par celui de colonne, tantôt par Encephalos, celui de rameau. Les raisons qui m'ont engagé à le faire repoussent sur la manière dont s'opère cette circulation. En effet, le sang chassé dans la partie antérieure du teste paraît s'y répandre et s'y disséminer de ces parties, plutôt que d'être contenus dans des vaisseaux particulières. Je ferai cependant observer qu'il existe dans ce liquide quatre espèces de courants qui ferment les quatre rameaux dont j'ai parlé plus haut, et que, dans les nœuds comme dans la queue, la circulation ne se fait pas d'une manière aussi diffuse que dans la partie antérieure du teste, le liquide globuleux paraissant y être renfermé dans une espèce de large canal pratiqué dans le parenchyme de ces parties.

It was probably owing to the misinterpretation of the remarkable facts which Cuvier had observed, that Meckel, notwithstanding the analogous structures, in the meanwhile pointed out by Jurine in the Argulus, and by Gaspard in the Helix pomatia, was led to deny the existence of the apertures of communication observed by Cuvier in the large muscular vena cava of the Aplysia. In 1832, however, the writer of the present article detected a structure in the venous system in the Nautilus pompilius closely analogous to that which Cuvier had pointed out in the Aplysia; but, having traced the continuity of the proper lining tunic of the great muscular vena cava, through the apertures in that coat, with a similar membrane lining the abdominal cavity, he was led to describe the tunic of the abdominal sinus as the peritoneum. "There are several small intervals left between the muscular fibres and corresponding round apertures in the membrane of the vein and in the peritoneum, so that the latter membrane is continuous with the lining membrane of the vein." And, after referring to the analogous structure in the Aplysia, the author adds, that this correspondence leads to the "suspicion that it may be more generally found on a further and more diligent investigation of the venous system in this remarkable class of animals."

Ten years later, M. Pouchet, professor of Zoology at Rouen, demonstrated in Limax, a structure answerable to that which M. Gaspard had described in Helix, viz., that the blood passed from the arterial capillaries into the visceral cavity, whence it was received by particular orifices into the veins that carried it directly to the pulmonary chamber, ramifying there like a vena porta before returning to the heart.

In 1834, the same year in which were published the figures and descriptions by Hunter of the circulating system in the lobster, M. Milne Edwards recorded his examination of the same system in the same species, in the Histoire Naturelle des Crustacés, vol. i., p. 101. He there describes the expanded venous sinuses as "plutôt des lacunes situées entre les divers organes que des canaux à parois bien formées." (Op. cit., p. 102.) The term "lacunes" is also adopted for that of "venous sinuses" by the editor of the posthumous edition of the Leçons d'Anatomie Comparée de Cuvier, t. vi., 1839, pp. 504, 505.

In 1843 M. Quatrefages believed that he had discovered a Mollusk, his Eolidina paradoxes, in which the organs of circulation were reduced to a univentricular heart and a system of arteries. "Le système veineux," he writes, "manque entièrement. Il est en quelque sorte remplacé par des lacunes du tissu aréolaire." In the same Nudibranchiate Mollusk the ramifications of the alimentary canal are also described as penetrating the branchiae, "where the chyle was directly submitted to the atmospheric influence exercised by the surrounding water." To this supposed condition of the digestive, circulating, and respiratory systems, he gave the name, "Phlebenterism," and proposed thereon some corresponding changes in the classification of the Mollusca.

In the Report on this and other memoirs of M. Quatrefages on the Nudibranchiate Mollusks, by M. Milne Edwards, that distinguished professor adopts the mode of interpreting the modification of the venous system, and applies it to the Crustacea. "Il existe (dans le genre Eolidine) un cœur et des artères bien constituées, mais pas des veines proprement dites, et le sang ne revient des divers parties du corps que par un système des lacunes irrégulières, disposition tout-à-fait analogue à celle dont les Crustacées nous avaient déjà fourni un exemple. Enfin dans d'autres espèces, que M. Quatrefages a découverte sur les côtes de la Bretagne, le cœur et les artères disparaissent à leur tour ; de sorte que la circulation devient des plus incomplètes et ressemblent à celle qu'on appercevoit chez les Bryozoaires." Subsequent researches by Messrs Embleton and Hancock demonstrated, however, the existence in the genera Eolis and Actonia, of both heart and arteries; and the same careful anatomists confirm the observations of M. Souleyer, that the veins were not wanting in the Nudibranchs, but had only undergone that modification of form to which the term sinuses is more properly given. Not any of these authors have, however, published exact and recognisable representations of the relations of the attenuated tunica propria of the veins, to the interspaces or laminae which that tunic lines in forming the large and irregular sinuses in which the blood is diffused.

In several of the writer's monographs, and especially in plate iii., fig. 1, of that on the organization of the Brachiopoda, he has adduced evidence in proof that the diffused and supposed merely lacunar venous system is continuous, i.e., forms a part, by continuity of tissue, with the rest of the circulating system. Thus all anterior researches rightly interpreted, since the time of Hunter, have served to confirm and establish the accuracy of that great anatomist's appreciation and explanation of the facts he discovered in insects and crustaceans, and to extend our knowledge of the same modification of the venous system as it exists in the series of the Molluscous animals.

CLASS IV.—PTEROPODA.

Encephalous Mollusks with wing-like fins from the sides of the head or neck.

The Pteropoda are so called on account of the resemblance of their principal organs of motion to a pair of wings, both as to form and in their mode of action on the surrounding medium. These expanded fins are not, however, the homologues of the part called the "foot" in Gastropods.

The Pteropoda are small marine floating Cephalous Mollusks, many of them of minute size. The greatest extremes in variety of form are presented in this order,—some species of Hyalea (fig. 31) and Euribia (fig. 40) being globular; others, as certain Cleodore (fig. 34), being very long and slender. The body is divided into a "somatal" and "visceral" part.

In some—e.g., Hyalea (fig. 31), Spiralis (fig. 42), and Cymbulia (fig. 39)—the head is not distinct from the muscular part of the "somata;" in others—e.g., Clio (fig. 45), Pseudomorpha (fig. 46)—it is defined therefrom by a constriction or neck. In the seemingly headless group (Thecosomata) the fins are confluent by their bases at the mid-line of the dorsal aspect; in those with distinct heads (Gymnosomata) the fins spring separately from the sides of the neck.

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1 Sur l'Argule fossile, in Annales du Muséum, tom. vii., p. 431, 439 (1806). 2 Recherches sur la Physiologie de l'Escargot des Vignes (Helix pomatia), Journal de Physiol. de Magdeur, 1822, t. ii., p. 295. 3 Owen, On the Fossil Nautilus, &c., 4to, 1832, p. 28. 4 Recherches sur l'Anat. et la Physiol. des Mollusques, 4to, Rouen, 1841. 5 Annales des Sciences Nat., 1843, t. xix.; Compte Rendus, 1843, p. 1124. 6 Observations sur les Mollusques Gastéropodes désignés sous le nom de Phlébenteres, par M. de Quatrefages, Compte Rendus des Sciences, &c., 1844, t. xix., p. 355. 7 Monogr. cit. Palaeontograph. Soc., 1853. 8 Gr. "wing-footed." As in most other natural groups of Cephalous Mollusks, some Pteropods are naked, others provided with shells; the former have the best developed and most distinct heads. The mantle presents corresponding modifications, or rather the part of the skin, properly so called, exists only in the testaceous Pteropods. In them the pallial fold is reflected forwards over the visceral mass, leaving a wide anterior opening for the access of water to the gills, which the mantle thus conceals; it sends out processes conformable to those of the shell. Its surface is ciliated. In the naked Pteropods the integument is confined to the surface of the animal, and is perforated only by the alimentary, excretory, and genital orifices. It is thick, and of a firm—sometimes almost cartilaginous (Euribita)—texture.

The shell presents as extreme varieties of form as does the body, but is always characterized by the delicacy and transparency of its texture in these little floating Mollusks. It deviates least from the ordinary form of the spiral univalve in the Pteropod, called on that account Spirialis. In this genus (fig. 42) the shell is discoid, with the last whirl much expanded, and umbilicate; the rest are disposed in from three to five whirls, projecting more or less from the last, with the mouth often angular; and in some (Sp. ventricosa) subcanaliculate the shell is sinistral. Some of the species (e.g., Sp. elathrata) are provided with an operculum (fig. 43). In Limacina the turns of the shell are reduced to one whirl and a half. In Hyalea, fig. 32, the shell resembles a bivalve in which the two valves have been cemented together along the hinge, leaving a narrow fissure in front and at the sides. In Cleodora, fig. 34, the shell is narrowed and lengthened out, the two plates being united together along the sides, so as to leave only an anterior aperture. In Cymbulia, fig. 39, the shell is internal, symmetrical, shaped like a boat or slipper, and of a gelatinocartilaginous texture. The cephalic ganglions (fig. 35, b, nervous system of Cleodora) are united by a commissural band a, passing above the oesophagus. The pedial ganglia e, and the "branchial" or splanchnic d, form a mass which is below the oesophagus.

No Pteropod possesses very distinct or well-developed eyes; but the acousticle, or little sac with calcareous crystals attached to the suboesophageal ganglions, exists in all, and was first discovered in these delicate transparent floating Mollusks.

The testaceous Pteropods have two tentacles; the naked ones have four. In these the vent is on the right side of the body, whilst in most of the testaceous Pteropods it is on the left side, the shell where spiral being sinistral. The heart consists of an auricle and ventricle, its position varying with that of the gills, which offer extreme modifications in this circumscribed group.

The organs of the male and female sex are combined in the same individual. The most essential part of the complex generative organs of hermaphrodite Cephalous Mollusks, which had been alternately described as testis and ovarium by different authors, before accurate microscopic observation had proved that both organs were combined in the same body, will be called in this article the "ovispermal" gland or body. The generative orifices are on the right side, save in Cymbulia, where that of the penis is median; the organ so called is for excitation, being imperforate, and is remote from the sperm-outlet.

ORDER I.—THECOSOMATA BL.

With an external shell; head indistinct

Genus.—Hyalea Lamarck.

Sp. Hyalea tridentata (figs. 31, 32).—The shell of Hyalea Pteropoda consists of two plates, which, like the valves of a Brachiopod, are "dorsal," f, and "ventral," g, in relative position. It bears, indeed, such a resemblance to a Bivalve, that Forskål, its discoverer, referred it to the genus Anomia, as An. tridentata. To Cuvier belongs the merit of having determined, by anatomical investigation, the pteropodal affinities of the beautiful little Mollusk, for which Lamarck, after Lamarck's remarks on the affinity of the shell to that of Brown's Clio, proposed the name of Hyalea.

The animal consists of a "somatal" (fig. 33, C, d) and "visceral" (g, h) division. The former is a broad, depressed muscular part, from which the head is not distinct, expanding on each side into a fin C, with a trilobed margin, the fins being united by a middle plate or lobe d (fig. 33), continued from their hinder border across the ventral aspect of the body. The head is indicated by two small clavate tentacles (fig. 33, b), and by the mouth a, which opens on the ventral aspect, and is provided with two labial appendages (fig. 32, f). The visceral division is lodged in the shell; the mantle opens in front, and sends off from its sides variously-shaped extensible appendages (figs. 31 and 33, e). The vent (fig. 33, o) is on the left side of the body, near the anterior border of the mantle and the left extremity of the gill. The vulva y is on the right, behind the base of the right fin; the male outlet is upon the right cephalic tentacle b. The gill (fig. 33, ii) is horse-shoe-shaped, and circumscribes the visceral mass behind and at the sides, being quite concealed by the mantle. The shell (fig. 32) is vitreous, transparent, and fragile; symmetrical; more or less globose, chiefly by the convexity of the ventral valve g; it is tridentate behind, with the angles or teeth h (figs. 31 and 32) more or less produced, according to the species, with a fissure-like opening i, anteriorly and laterally, not adapted for closure by an operculum. A single retractor muscle (fig. 33, k) arising from the median point or apex of the shell, passes forward along the dorsal aspect, expanding, and then bifurcating to spread abroad and interlace with the muscles of the fins. By the action of this retractor the whole "soma" can be retracted within the shell; the lateral prolongations of the mantle have their own contractile fibres for the same purpose.

For a full description of the nervous system of Hyalea the reader is referred to the Voyage de la Bonite, Partie Zoologique, tom ii., 8vo, p. 131, and Atlas, fol., pl. 9, figs. 20–24, whence the figures referred to are taken.

The labial boundaries of the triangular mouth are prolonged, and subside in the grooves between the fins and middle lobes of the soma. The glottidium has three diverging series of minute recurved teeth.

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1 Eydoux et Souleyet, Annales Francaises et étrangères d'Anatomie, tom. ii., p. 305. 2 Gr. for "glassy." 3 Fissura Arabea, p. 124; and Jones, tab. xi., fig. B. 4 Annales du Muséum, tom. iv. (1804), pp. 223–234. 5 Les nageoires ne forment qu'un seule expansion, l'analogue du pied des Mollusques gastéropodes," Souleyet, Zoologie de la Bonite, 8vo, p. 104; De Blainville, Malacologie, pl. 46, and Journal de Physique, tom. xliii., p. 81. The skin of the *Hyalea* is produced and reflected from behind forwards, so as to form a true mantle, which circumscribes beneath the viscera a branchial cavity opening anteriorly. The branchial plates are attached to the bottom of this cavity, and are disposed, as Cuvier well described, in a crescentic curve, almost circumscribing the viscera. Within the horns of the crescent are two supplementary series of gill-plates, parallel with those terminal parts of the main series, and more dorsal in position. The pericardium lies behind the left supplementary gill; the anus opens in front of it. The gill-plates are like those of the limpet, but are more complex in structure. The urinary sac surrounds part of the auricle, the pyriform ventricle sends off a short aorta, which divides into two main branches. The genital or ovispermal gland is transversely ridged or folded; its duct enters the side of a second larger canal, with glandular walls, one end of which is prolonged into a convoluted blind tube, the other terminates in a kind of matrix consisting chiefly of compacted convolutions of a tube. The duct or canal of the matrix (vagina) passes forward and to the right, to terminate behind the right fin.

The exciting organ is a muscular tube whose folds form a prominence to which the upper border of the somatal integument is attached; the tube opens externally in front and to the right of the mouth; its internal dilated end appears to terminate as a closed cavity, connected only with a retractor muscle. It is thus insulated, like the palp of the male spider and the intromittent organ in some Cephalopods, from the remaining and more essential part of the male apparatus.

The *Hyalea* inhabit the warmer temperate and tropical zones, and have been taken in the Atlantic, Indian, Pacific, and Mediterranean seas. They swim with moderate speed by the action of their fins, but always with the back downwards, which is indicated by the deeper colour of the more exposed convex ventral valve. More than twenty species have been defined, of which the *Hyalea tridentata* is here selected for the special illustration of the genus. The *Pteropoda*, beautiful translucent shell of this species, about 9 lines in length, is of a reddish-yellow, shading off to rose colour, but pale on the dorsal plate. The ventral valve is marked anteriorly by fine transverse striae. The dorsal valve has five convex aorta, which diverge forwards.

**Genus Cleodora**, Péron, Lesueur. A beautiful transition from *Hyalea* to *Cleodora* is made by the *Cleodora lanceolata*, Lesueur, in which the anterior borders of the shell are so produced as to make its fissured opening in part lateral as well as anterior; the productions of its apex and sides also resemble those of *Hyalea tricuspidata*. Other *Cleodora* gain in length and lose in breadth, until the opening of the shell is wholly anterior, and its cavity prolonged into the tubular or serpiform shape presented by the *Cleodora aciculata* (fig. 34).

The mantle sends out no lateral processes or enveloping folds in such *Cleodora*. The fins are simply notched or bilobed, and are more distinct from the median fold. The mantle forms a pouch for the branchiae like that of the *Hyalea*; its muscular border follows closely the progressive modifications of the shell aperture, and becomes simply circular anteriorly, as at e in the species figured; it is beset with tufts of vibratile cilia, as in *Hyalea*. Save in the modifications of proportion, chiefly of length, conformably to the altered form of the body, the digestive organs closely resemble those of *Hyalea*. In fig. 35, a is the buccal mass, l the gullet, n the stomach, p the liver, and o the intestine, which terminates further from the free border of the mantle than in *Hyalea* at o. The gill forms half of a longer and narrower ellipse than in *Hyalea*; the heart has a similar relative position, and consists of the usual auricle and ventricle, but the urinary cavity is a more distinct pyriform sac than in *Hyalea*. The ovispermal gland (fig. 35, u) is sub-bilobed and transversely plicated. The oviduct is continued directly into the glandular tube v, which is not prolonged into a caecal appendage, but contracts as it terminates in the matrix w. The genital outlet y is behind the right fin; the excitory organ is disconnected with the essential part of generation, and is more in advance. The retractor muscle k closely resembles that of *Hyalea* in its origin and insertions.

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1 Many of which are beautifully figured in the *Voyage de la Bonite*, plates 4, 5, and 6. Pteropoda. Cleodora from Hyalaea, it is still more so to discriminate from Cleodora the more slender conical species which have been removed from that genus under the names Styliola and Creseis.

In the Cleodora cuspidata and Cleodora pyramidata the shell is pyramidal, three-sided, striated transversely with the mid-costa, in the dorsal valve, so produced as to give it a keeled appearance. In Cleodora virgula and Cleodora aciculata the shell shows neither aortic nor striae, and is truncate anteriorly. The fins have a small lobe on their inner border.

The geographical distribution of the species of Cleodora agrees with that of Hyalaea.

Genus Cuvieria.1 Rang.—The Pteropods of the genus Cuvieria are closely allied to the Cleodora, and were ascribed to that genus by their discoverer M. Gaudichaud (as Cleodora obtusa and Cleodora rosea). The fins (fig. 36, e) have the same elongated form and terminal notch; but the intermediate lobe d, which some suppose to be homologous with the part f, fig. 67, in Carinaria,2 presents also a notch at its middle part.

The Cuvieria are also more particularly distinguished by an appendage, which is attached to the under part of the neck by a pedicle, and divides into two lobes, one (fig. 36, e) elongated and pointed; the other, f, expanded, and with a thickened, wavy, or plicated border. From the lower part of the pedicle a fold of skin connects the terminal orifice of the vagina with the neck; and the bifid appendage is probably the homologue of the penis in Heteropods. The mantle does not develop lateral leaflets, as in Hyalaea.

The crescentic branchia (fig. 38) is placed deeper in the Pteropods. mantle-bag than in Cleodora, and its horns are less symmetrical. The shell (fig. 37) has a septum s, which partitions off its hinder end; and its aperture i has a rounded, not a trenchant border. M. d'Orbigny affirms that the hind end g is originally pointed as in Cleodora, and that its usual truncation is due to accident.

In fig. 38, i is the branchia, r the auricle, s the ventricle, and t the pyriform urinary sac.

The Cuvieria range from the tropics to the latitude of Cape Horn. They are less common than the Hyalaea or Cleodora, but their mode of life is the same.

In the Cuvieria columnella (figs. 36 and 37), the shell is subcylindrical, elongated, smooth, contracted, or truncate at the hind end g; depressed anteriorly, with an oblique aperture i with rounded borders, the dorsal longer than the ventral lip.

Three varieties of form of this shell have been noticed: in the first two the shell is moderately thick, firm, hyaline, sometimes of a feeble rose colour, and with fine longitudinal striae; in the third variety the shell is thin, fragile, and quite transparent. The latter variety occurs in the Chinese and Pacific Oceans.

Genus Cymbulia, Péron and Lesueur.—In this genus the cartilaginous shell is developed in the substance of the mantle, but is distinct. It is covered by so thin a layer of the mantle that this is often torn, and the animal loses its shell, which slips out; its substance contains chitine. It is transparent, skiff-shaped, symmetrical, pointed in front, truncate behind, with the aperture elongate and ventral.

The body of Cymbulia Péronii (fig. 59) is somewhat obscurely divided into a somatal and visceral part, the latter only being lodged in the shell. The fin-like expansions of the soma, C, C, are unusually large, expanding to beyond the hinder end of the body.

The mouth opens on the ventral surface of the body, at the anterior interspace between the fins. Above and in front of this orifice are two prototacies, in front of which, at the middle line, is the prepuce.

The visceral part is not inclosed by a pallial sac opening anteriorly, as in Hyalea, but is covered by the mantle only dorsally; while on the ventral side it rests in a sac, formed by the detachment of the skin from the dorsal surface of the fins, and opening posteriorly. There are no proper or free "borders of the mantle;" that which circumscribes the orifice of the

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1 In honour of Cuvier, the founder of modern Malacology. 2 "Fins united by a semicircular lobe, the equivalent of the posterior element of the foot." (Woodward's Manual, p. 204.) Pteropoda visceral cavity is a simple fold, which is reflected thence over the shell, which thus becomes "internal." The gills are two membrano-vascular plates on the sides of the visceral pouch.

Sp. Cymbulia Peroni, Lam.—This is the type species, and is met with in certain parts of the Mediterranean.

Two other species have been characterized as Cymbulia punctata and Cymbulia Norfolkensis by the naturalists of the "Astrolabe." These Pteropods swim, like the Hyalea and Carinaria, with the belly upwards. Mr Adams figures a Cymbulia proboscidea.

Genus Tiedemannia, Delle Chiaje.—The Pteropods of this genus are Cymbulia, with the fins and foot-process confluent and forming an orbicular disc; the mouth is at the apex of a long proboscis; the tentacles are elongated and connate; the eye-tubercles minute. The shell is hyaline, gelatinous, slightly excavated; it appears to be soon or frequently shed.

Sp. Tiedemannia Neapolitana, V. Beneden (Exercises Zootomiques, p. 21, pl. 2).—In this species the proboscis is slender, and the fins have white and yellow spots at the margin.

Genus Euribia, Rang.—The genus Euribia is founded on a small and very rare Pteropod, of the shape and about the size of a pea. (See the figure opposite the left hand in fig. 41.) The chief part of the integument, although transparent, is of almost cartilaginous consistency, but anteriorly becomes suddenly thin and flexible, forming a fold, which borders on oblique fossa into which the "soma," or the fore part of the animal developing the fins and tentacles, and covered by a thin and extensible skin, can be retracted with all its appendages. The firm globular hind part of the integument, which thus acts as a shell, retains so much muscularity that it can draw one side of the anterior depression towards the other, and thus close the seeming aperture upon the contracted and retracted cephalic appendages, as in the enlarged figure opposite the right hand in fig. 41. Of these appendages the largest and most conspicuous are the fins (fig. 40, c, c), which are attached to the sides and under part of the head, meeting and partially blending at the latter aspect; but with a median pedial appendage f at the interspace. The fins curve backwards, and are dilated at their extremity.

Anterior and ventrad of the fins come off the long, transversely-striated, and pointed appendages (fig 40, e, e), and in front of these are the tentacles l, l. At the fore part of the right fin is the preputium; at the back part is the genital or vaginal orifice y. The vent o is to the right of the pedial appendage f. The mouth a is an elliptic aperture, with two labial palps or tentacles d. The "glottidium" has a biserial dental plate. The pair of cecal salivary glands; the large stomach surrounded by the liver p; the reflected intestine o; and the androgynous generative organs u, v, w—all accord with the pteropodal type. The retractor muscles of the soma are shown at k, k in fig. 35.

The Euribia Gaudichaudii, Eyd. and Soul., was captured by the accomplished naturalist after whom it is named in the Pacific Ocean, in 20° N. Lat. and 170° E. Long. He noticed in the living animal the vibratile action of tufts of cilia on the anterior border of the fins. These appendages were of a pale rosy tint; the visceral mass was partly brownish, partly of an orange-red.

The Euribia Norfolkensis (fig. 41), captured by MM. Quoy and Gaimard, off Norfolk Island, Australia, differs by the rugosities or minute tubercles which roughen the firm integument. Fig. 41 shows this part in its retracted and contracted state magnified; the figure to the left shows the aperture of the quasi-shell, a little widened, of the natural size.

The small Pteropod which forms the genus Psyche of Rang appears to differ from Euribia only in the softer consistency of the integument. The name Psyche, moreover, is pre-appropriated by the Lepidopterists.

Euribia is in some measure intermediate between Cuveria and Pneumodermon, and seems to indicate the transition from the naked to the shelled Pteropods.

Genus Spirialis, Eydoux and Souleyet.—Were the long and slender shell of Cleodora to be twisted spirally, and its anterior aperture widened and closed by an operculum, it would become transmuted into a Spirialis (fig. 42). The soma and its natatory expansions differ only by slight and unessential modifications of form from those of Hyalea and Cleodora. Thus the fins (fig. 42, C, C), are elongated, slightly expanded, and rounded at their termination. The intermediate lobe is sub-pedunculate, and bears the operculum f on its upper surface. There are two minute tentacles. The prepuce is at the front border of the head, a little to the right. The proper genital opening is behind the base of the right fin. The branchial sac is now dorsal in position, as in Gastropods, through the spiral twist of the body. About half a dozen species of Spirialis have been defined.

The Spirialis Bulimoides (fig. 42) has an elongated sinistral shell of six whorls, with a sub-acute spire. The mouth is ovaloid, angular at the fore part, with the labrum sharp; the columella is slightly arched. The shell scarcely exceeds a line in length; it is smooth. The operculum (fig. 43, f) is chitinous, dextral, and paucispiral.

In the Spirialis clathrata (fig. 44) the surface of the shell shows slightly elevated striae, decussating so as to intercept lozenge-shaped areolar. It has three sinistral whorls g, with a capacious ventricose last turn or chamber f. The minute Pteropods of this genus are abundant, and are widely diffused through the warm latitudes of the ocean.

Genus Limacina, Cuv.—This genus was proposed by Cuvier for the Clio helicina of Phipps, and probably the generic name was given, as Van der Hoeven suggests, inad-

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1 Lat. for a "small slug." 2 Voyage towards the North Pole, 4to, 1774. Pteropoda, vertently for Helicina; the slug-like Clio being distinguished as the Cl. Limacina from the testaceous Pteropod now generally recognised as the Limacina arctica.

Limacina has a fragile, discoid, sinistrally spiral shell, deeply umbilicate on one side, and with the spire slightly projecting on the other. It differs from that of Spiralis in having no operculum.

Sp. Limacina arctica, Cuv.—This species exists in myriads in the arctic seas.

Sp. Limacina antarctica, Hooker.—The species in which the shell-whorls are transversely striated, with the fins notched on the hinder border, and with the post-pedal lobe emarginate, is equally abundant in the antarctic seas.

Order II.—GYMNOSONATA, Blainville.

Without a shell; head distinct; fins attached to the sides of the neck.

Genus Clio, Linn. and Müller.—The earliest form of Pteropod known to naturalists was a species of the genus to which the above name, originally applied by Brown to another form, and by Linnaeus to all the Pteropods he knew, is now by common consent restricted. This species (Clio borealis, Gmel., fig. 45), the well-known whale's food, or "whale-bait" of the arctic voyagers, is remarkable for the high northern latitudes in which it exists, and for the myriads of its individuals that find subsistence in those icy seas. First indicated by Martens in his Voyage to Spitzbergen (1675), and afterwards described, as to external characters, by Pallas; its true nature and affinities were made known anatomically by Cuvier. Eschricht and Gegenbauer have ably completed that work, and a wonderful structure has been unfolded through their patience and skill. A study of the works cited below will well repay the earnest student of Molluscous organization.

In the genus Clio the body is oblong, acuminate, or appendiculate behind (Clio longicaudatus). The head (fig. 45, a) is defined by a constriction (Clio borealis), or by a neck (Clio longicaudatus); it is terminated by two cowl or hoods (preputia, Pallas), containing each three appendages (fig. 45, a'), and by a pair of retractile tentacles b. The fins c are more wing-shaped than in most Pteropods; their contracted bases are confluent across the ventral aspect of the "neck." In swimming, the Clio brings the ends of its fins almost in contact, first above, then below. The rudiment of the foot d is bifid. At the back of the neck are two black "ocelli." The preputium in front of the right fin gives exit to an unusually long bent excitory organ z, when it is unfolded and everted. The vaginal orifice is behind the right fin. The vent o is still more posterior and nearer the ventral surface. The conical appendages of the head, when fully expanded, form a radiated crown;

they are of a red colour on the recent animal; which colour Pteropoda depends on the presence of numerous separate superficial points, about 3000 on each appendage. Each point, microscopically examined, is a sheath inclosing a central body composed of a stem terminated by a tuft of about twenty pedunculated discs. Eschricht, their discoverer, calls them suckers, and reckons up about 360,000 of them. Gegenbauer suggest that they may be pedunculated epithelial or pigmental cells or scales.

Sp. Clio borealis, Müller (fig. 45).—This species abounds in the neighbourhood of Greenland and Spitzbergen, and though scarcely an inch in length, constitutes, with its common associate, the still smaller Pteropod, Limacina arctica, the chief food of the great whalebone whale (Balaena mysticetus).

Sp. Clio fusiformis, Quoy and Gaimard (Zoologie du Voyage de l'Uranie, pl. 66, fig. 2): Clio longicaudatus, Eydoux and Souleyet (Zoologie du Voyage de la Bonite, pl. 14, figs. 19-21).

Genus Pneumodermon of Cuvier.—A naked Pteropod, about an inch in length, discovered by Péron in the Atlantic, and submitted to Cuvier's examination, was described by the great anatomist in the fourth volume of the Annales du Muséum, 1804, under the name of Pneumodermon, or "lung-skin," from the circumstance of the gills (fig. 46, f) being distinctly developed from a part of the integument. Other species of this genus have since been discovered.

The head (fig. 46, a, and 47, A) projects anteriorly and distinct from the fins c, e; the mouth, when the mouth-mass aa' is retracted, is a small terminal vertical slit, on each of which is a pit with a small pro-tentacle. The post-tentacles are still smaller, and are bifid. On the right side, in advance of the fin, is the preputial orifice; near the hind border of the fin is the vaginal orifice (fig. 47, y). Still further back, but still on the right side, is the vent (fig. 46, o'). On the middle of the ventral surface of the anterior constricted part of the body is a heart-shaped disc (fig. 46, d), composed of two lateral plates united together and attached along the mid-line, but elsewhere free; from above the back part or base of the above disc a long triangular lobe (fig. 46, f') freely projects backwards. These appendages are homologous with the "fin-foot" and "tail" in Heteropods; like the foot-disc in which the Pneumodermon uses the discoid part (fig. 46, d), to attach itself to the sides of the glass in which it may be placed.

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1 Spicilegia Zoologica, fasc. x., p. 28, tab. i., figs. 18, 19. 2 Anatomische untersuchungen ueber die Cliona Borealis, Copenhagen, 1838. 3 Unter suchungen ueber die Pteropoden und Heteropoden, Leipzig, 1845. 4 Cet appendice est tout à fait analogue, par sa position et par sa forme, au pied des Mollusques Gastropodes—ses usages paraissent être semblables à ceux du pied des Atlantes, des Piroules, &c., qui, comme on le sait, est transformé en ventouse dans une partie de son étendu. (Zoologie de la Voyage de la Bonite, tom. ii., p. 257.) The interesting homology above enunciated by MM. Eydoux and Souleyet, referred to in another part of Mr Hauck's Memoir (p. 20), is adopted by that writer, who informs the Royal Society,—It is very remarkable that Cuvier should not have recognised in the "spécie de menton" and the "deux petits lèvres" of Pneumodermon, the homologues of the foot of the Gasteropoda. ("On the Anatomy of certain Pteropoda," Phil. Trans., 1853, p. 39.) The true discoverers of this homology, with more candour, point out the belief which Cuvier entertained of the existence of a fissure between the folds of the little foot, as the ground for his describing them as "labial" folds. The anatomical illustrations of the Pteropoda here given are chiefly derived from MM. Eydoux and Souleyet. The visceral division (fig. 47) forms the seeming trunk of the animal; it is expanded, ovoid, and terminated behind by membranous plates which support the branchial filaments. There is also a vascular process (fig. 46), where the integument is very thin, from the right side of the body a little in advance of the terminal branchial region, which process is most intimately related to the vascular and respiratory function.

The only part of the integument that can be properly regarded as "mantle" is the slightly projecting border (fig. 46) from the four-radiate respiratory surface; elsewhere the skin is perforated only by the oral, excretory, and genital outlets.

On each side the buccal cavity is an invertible pouch (fig. 47, a), to the surface of which are attached numerous pedunculated suckers (fig. 47, b, and fig. 46, l). The glottidium is notched before and behind, and each moiety supports four rows of recurved denticles. Beyond the glottidium are the orifices of two long accessory appendages, consisting of an outer sheath (fig. 47, e) having a muscular coat of longitudinal and circular fibres, and an inner tube lined by an uncinate membrane, capable of being exerted and protruded (as at a, fig. 46) when the hooks become external. The oesophagus has a sigmoid flexure by which it adapts itself to the varying conditions of the buccal appendages. The salivary cecal glands (fig. 47, f, f) lie by its side. The stomach m is capacious, and seems to be a lining to the hepatic mass p; the intestine o is short, and is reflected forward to terminate on the right side (fig. 46, o'), some way in advance of the respiratory surface of the mantle. This surface is at the posterior part of the body (fig. 46, i.e), which shows four curved membranous folds; the dorsal and ventral pair turn their convexities to each other, and are united by the shorter lateral folds, which circumscribe an irregular quadrangular space; the longer folds have a pinnate arrangement of branchial lamellae. On the right side of the body there is a process of the skin also supporting branchial lamellae.

The heart, consisting of an auricle r, and a ventricle s (fig. 47), is situated at the back part of the visceral cavity, close to the part whence the last-mentioned branchial process projects. The aorta divides and sends one branch to the visceral mass, the other to the soma or rudimental foot and its appendages. The renal sac q, which Lovén deems homologous with the plicated auricles of Terebratula, is closely connected with the venous trunk entering the auricle in Pneumodermon. The male and female organs are combined as in other Pteropods. In fig. 47, u is the blended testis and ovarium, v' and w are the combined oviduct and sperm-duct, v is the uterus, x the vagina, and y the genital orifice.

Sp. Pneumodermon Peroni. Cuvier (fig. 46).

This species is of a violet-brown colour, deepening towards the head; the fins and branchial membranes are white; it is about an inch in length. The species is common in the Atlantic, in tropical latitudes. The individuals are aggregated in large groups; they swim swiftly, and in their natural position, with the ventral surface downwards, not reversed like the Hyalaea. When at rest they can attach themselves pretty firmly to any floating body by their rudimental foot, or by their buccal suckers.

Professor Müller describes the larva of Pneumodermon as being girt by three bands of vibratile cilia, one round the fore part, another round the middle, and the third round the hind part of the body.

CLASS V.—GASTEROPODA.

The Encephalous Mollusca grouped together under the above name by Cuvier, include the most typical forms of the province; yet can hardly be regarded as so natural a class as either the Brachiopoda, Lamellibranchiata, or Cephalopoda. The muscular disc for creeping (fig. 48, dd), which is developed from more or less of the ventral surface of the body, is the common external character by which the Gasteropods are associated together; and yet there are species (fig. 71, e.g.) in which the ventral foot is as little developed, or recognisable as such, as its homologue is in Pneumodermon and some other Pteropoda.

The snails and slugs exemplify the ordinary mode and characteristic instrument of locomotion in the Gasteropodous

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1 Gr. for "belly-foot." class, most of the members of which are unsymmetrical, with the visceral division of the body coiled spirally and protected by a shell; the breathing organs of the left side being, as a rule, atrophied. Most Gastropods are marine; some inhabit fresh waters; a few are terrestrial. The species offer corresponding conditions of the respiratory organs in relation to those media, with minor modifications, of which the systematic naturalist has availed himself in distributing the numerous and diversified members of the class into orders.

The air-breathing Gastropods (Pulmonata) constitute a tolerably natural group: those that breathe water are much more diversified.

In certain small shell-less marine genera,—e.g., Rhodope, Tergipes, Eolidina,—no distinct respiratory organs have been detected; these form the order Apneausta.

Some that breathe by gills have those organs exposed. The genera which support them on the back, such as Glaucon, Scyllaea, Tritonia, form the order Nudibranchiata; all the species are without shells in the mature state. Those genera which carry the gills at the lower part of the sides of the body, between the foot and mantle,—as, e.g., Phyllidia,—constitute the order Inferobranchiata; they are likewise naked when mature. The genera in which the gills have a similar position, but extend around the body, as in the limpet (Patella), and in Chiton, form the order Cyclobranchiata; they are protected by a conical shell composed of one or of many pieces.

In the rest of the water-breathers the branchia are concealed. Those genera,—as, e.g., Aplysia and Bulla,—which have the gills protected by a fold of the mantle containing a rudimental shell, or by a reflected portion of the foot, form the order Tectibranchiata.

In all the foregoing orders of Gasteropoda the male and female organs of generation are combined in the same individual.

In the remainder of the class the sexes are distinct. A small order of marine Gastropods, in which the gills are packed in small compass, with the heart in a dorsal mantle-chamber or in a small symmetrical shell, is called Nucleo-branchiata; and also, on account of their stunted foot, Heteropoda. Another small group of marine Gastropods, including Fissurella and Halyotis, which have their comb-like branchia protected by a wide shield-shaped shell, is called Sentibranchiata. A third small group in which similar branchiae are protected with the entire body by a tubular shell, is called Tubulibranchiata. In the last, highest, and most extensive order, called Pectinibranchiata, from the comb-shaped gills, which have a special pallial cavity at the fore part of the back, and which, with the rest of the body, are protected by a spiral univalve shell, the males are provided with an introverted organ. A few species of Cymba, Litorina, Patudina, and Helix are ovo-viviparous; most Gastropods are oviparous. The young of the water-breathing Gastropods are excluded with a protecting operculated shell, which in the "naked" species is either shed or concealed by a fold of the mantle. They swim by means of a pair of ciliated fins attached to the sides of the head, and thus move far away from their inactive or sedentary parents. The larvae, as they may now be called, of all the water-breathers are very much alike, and undergo metamorphoses in the course of attaining their adult, nudibranchiate, nucleo-branchiate, or pectinibranchiate forms. The air-breathing Gastropods undergo no such metamorphosis; their shell commences by the deposition of crystals of carbonate of lime in the substance of their skin, above the visceral cavity, and it is persistent.

The soft parts of Gastropods are immediately invested by a soft inarticulated lubricious integument, forming in most a sub-circular fold (fig. 48, i) about the neck, behind which it is produced into a sac containing the circulating,

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1 Lat. for "having lungs." 2 Gr. for "without lungs." 3 Lat. for "roofed gills." 4 Lat. for "low-placed gills." 5 Lat. for "exposed gills." 6 Lat. for "shielded-gills." last-formed part. It is to this periodical growth of the mantle, and plethoric condition of the calcifying vessels, that the ridges on the exterior of the shell in the ventletrap (Scolaria pretiosa) and harp-shell (Harpa ventricosa, fig. 85) are due. Should the margin of the mantle, instead of being uniformly extended, send outward a number of detached tentaculiform calcifying processes, these will form a row of spines corresponding in length and thickness to the softer parts on which they are moulded; and as the calcification of the processes proceeds, the spines which were first hollow become solidified, and finally soldered to the margin of the shell.

This development of pallial calcifying processes or filaments, and of the resulting spines, likewise alternates with periods of the ordinary increase of the shell; and thus its exterior surface may become bristled with rows of spines, as in the Murex crassissima. The periodical excretion of the excess of calcareous matter in the blood is greatest in the carnivorous Univalves.

The simple form of univalve shell is the cone, which may be much depressed, as in the genus Umbrella (fig. 58), or extremely elevated and contracted, as in Terrellium (fig. 89), or of more ordinary proportions, as in the limpets (Patella). The apex of the cone is always oblique and eccentric; directed, in limpets, towards the head, but in other Gastropods towards the opposite extremity of the body. The conical univalve shell is generally spirally convoluted, sometimes in the same plane,—e.g., Planorbis,—but more usually in an oblique direction, as in Triton (fig. 50). The apex of the shell \(a\) is formed by the nucleus, or the part which was developed in the egg; it is mammillated in Fusus antiquus. The spiral turns of the shell are called "whirls," the last, \(w\), \(ae\), being the "body-whirl." The lines or grooves formed by their junction are the "sutures." The "whirls" above the body form the "spire" of the shell, \(pe\) to \(a\).

As a general rule, the spiral Univalve, if viewed in the position in which its inhabitant would carry it were it moving forwards from the observer, is twisted from the apex downwards from left to right, the spire being directed obliquely towards the right; but in a few genera,—e.g., Clausilia phyra,—the shell is twisted in the opposite direction, when it is called "reverse" or "sinistral." Some individuals of Bulinus partula, and Pupa, and a few marine species, as Fusus sinistrostrum, are sinistral. The part around which the spiral cone is wound is termed the "columella;" it is exposed by removal of part of the shell in fig. 50, \(o\). This central pillar is sometimes simple, sometimes grooved, sometimes plicated; in some shells it is solid, in some hollow, as in Solarium and Doliva (fig. 49), where the narrow elliptical aperture of the columella is seen to the left of the wide shell-aperture; it is termed the umbilicus. In Solarium, as in Philippia, the apex of the shell is inverted, and can only be seen by looking into the umbilicus.

The wide aperture which forms the base of the spiral Univalve is bounded by an "outer-lip" (fig. 50, \(pe\), \(ae\)) and an "inner lip;" the latter offers a smooth convex surface, over which the foot of the Gastropod glides to reach the ground. In many Univalves, including most vegetable-feeders, the aperture of the shell is entire; in others it is interrupted, the left side being formed only by the "body whorl;" or the "peristome" (as the margin is called) may be broken by a notch, like that which separates the outer lip from the umbilicus in fig. 49, or it may be perforated by one or more holes, or a portion of it may be produced into a canal or siphon (ae, fig. 50); this is sometimes termed the "anterior canal," and the notch or hole at the opposite end of the peristome is called the "posterior canal" \(pe\). These modifications are important, on account of the constancy of their relations to certain conditions of the respiratory organs. Thus all the Pectinibranchiate Gastropods, in which the water is conducted to the shell by a muscular tube or siphon, have the margin of the aperture either notched or produced into a canal; and the posterior channel (fig 50, \(pe\)) is anal in its function (Triton, Strombidae). Sometimes it is represented by a slit (Scissurella), or it is a tube (Typhis), or a perforation (Fissurella), or a series of holes, as in Haliozis.

The relations of these modifications of the univalve shell, which anatomy has made known, enable us to judge, from a fossil shell, of the nature of the medium of existence, of the respiratory medium, the food and habits, of its extinct constructor. The Gastropods which first appear in the Palaeozoic strata have entire mouths; the siphonated species are not found lower than the Lias, and they go on increasing in numbers in and from the Tertiary series to the actual sea-shores.

In some of the Gastropods the shell consists of one piece, as in fig. 58, when it is termed an "inopercular Univalve;" but the aperture of the shell, in the majority of the species, is closed by a lid or plate, attached to the back of the foot, and called the "operculum" (fig. 86, \(f\)). This lid is sometimes calcareous, forming a second shelly plate, but it more frequently consists of an albuminous membrane only, or is horny, thus presenting the condition which the shell itself manifests in certain genera, as Limax and Aplysia; the inner surface is marked by a muscular impression, but unlike that on the shell, and whose lines bear no relation to the external lines of growth. Some opercula increase by the addition of matter to their entire circumference, and these are either "concentric," as in Patulina, or "eccentric," as in Ampullaria and most of the Pectinibranchs. Other opercula grow by the addition of matter to part of their circumference, and these are either "spiral" or "imbricate." Spiral opercula may be either "paucispiral" (Litorina, fig. 52), or multisprial (Trochus, fig. 55). In the "imbricate" or lamellar opercula (Purpura, fig. 54), the layers of growth succeed each other in a linear series, and the nucleus is marginal. When the nucleus is apical or in front, as in Turbinellus and Fusus (fig. 53), it is said to be "unguiculate" or claw-shaped; when it has a projection, as in Nerita (fig. 51), it is "articulate." No operculum presents an annular form. Deshayes figures the operculum of Solarium patulum as composed of many distinct and spirally-disposed lamellae. Mr E. Layard has discovered a similar complex operculum in the Cataulax Austenianus, a little Univalve of Ceylon. In Torina straminea the operculum is thick, horny, circular, at the base, which is as large as the mouth of the shell, and concave externally. The rest of the operculum is produced conical, and of many whorls (fig. 56).

As the operculum sometimes varies in structure in the same genus, being horny in some species and shelly in others of Ampullaria and Natica,—as it is present in some volutes, cones, mitres, and olives, and absent in other species of these genera,—and as some genera in a natural family, e.g., Harpa and Dolium, among the Buccinoids, are without an operculum, whilst the other genera of the same family possess that appendage,—it obviously affords characters of secondary importance in classification. In Lithedaphus (Calyptraea) eques, the whole base of the foot secretes a calcareous plate, which is cemented to the rock; and the shell appears to consist of two valves. In the Chiton (figs. 72 and 73), the shell is divided into eight symmetrical pieces, arranged like scales upon the back. The first of these is the smallest, the last the longest, and most approaching the circular form. In the interior of the shell the muscular impression is usually crescentic, with the horns turned towards the head of the animal.

Most univalve shells are composed of three strata, which differ in the arrangement of the calcareous particles. The innermost layer is nacreous in the fucivorous Univalves,—e.g., Patella, Haliotis,—and resembles enamel in the marine species. In Cassis rufa each layer is composed of many laminae, which are perpendicular to the plane of the main layer; and each lamina consists of a series of prismatic cells, adherent by their long sides. The laminae of the outer and inner sides are parallel to the lines of growth, while those of the middle layer are at right angles to them. In the cowries (Cypraea) there is an additional layer, which is nacreous, and formed by the overlapping mantle-lobes (fig. 48, h, k) when the animal has attained its full growth. Such shells are called "cameo-shells," these ornaments being formed by the removal of one layer, and the carving of the next. Hunter discovered that the molluscous inhabitant of a shell had the power of absorbing part of its dwelling. This property, which is now generally recognised, is well illustrated by the thinning of the parietes of the internal whorls of the cones and olives, from which two out of the three layers of which they were originally composed may be observed to have been removed. The absorption of shell is also illustrated by the removal or smoothing down of the spines of the rock-shells (Murex), as the growing whirl expands and overlaps its predecessor by the flattening of the inner lip of the mouth of the Purpurea, by the widening of the focal aperture of the Fissurella; and it gives rise to various other modifications in the form and structure of shell in the progress of growth.

Another change of form is due to the physical decomposition or destruction of a part of a shell during the lifetime of the inhabitant. This occurs to the apex of certain Univalves after the shell has been excavated by the original occupant in the widening and lengthening the shell to accommodate it to an increase of bulk. Such shells are said to be "decollated," as, for example, the Bulinus decolatus, Cerithium obtusum (fig. 79).

The inhabitants of univalve shells dispose in different ways of that part of their calcareous abode which they evacuate in the progress of growth. In the decollated shells the vacated space is walled off, prior to its abrasion, by the formation of a thin nacreous plate. In the Vermus gigas the vacated portion of the tube is retained, and successively partitioned off; a series of concave plates or septa being thus developed. In the Mogilus antiquus the posterior part of the shell, as the soft parts move forwards, is progressively filled up with a dense, solid, subtransparent, crystalline deposit of carbonate of lime.

Univalves which inhabit rocks upon which surf-waves are ever breaking have stronger and denser shells than those that live in calmer seas, or in sandy and muddy bottoms. The shells of air-breathing Mollusks, which, as a rule, are thinner than those of marine species, exhibit analogous effects of external influences. The shells of the burrowing Bulini, e.g., are colourless and subtransparent; but the shell is vividly coloured in the species inhabiting plains with scanty vegetation and much exposed to solar light. The shells are largest and thickest in the arboreal Bulini of tropical forests, which live amongst an abundance of decaying vegetable matter.

The part of the mantle which invests the viscera in the conchiferous Gastropods—that behind letter i in fig. 49—is smooth, thin, and subtransparent, resembling the sac of a hernia, which, with the viscera themselves, appears to have escaped from the common muscular integument of the body. This "visceral mass," as it is termed, is lodged in the upper part of the cone of the shell, the spiral turns of which it follows. The head (fig. 49, ab) and foot d of the animal can be protruded from the mouth of the shell, and be retracted within its last whirl, by the action of a muscle which has its fixed point in the columella of the shell. This retractor, which is attached to the operculum, seems to answer to the posterior retractor of the foot in Bivalves, much better, at least, than to the great adductor muscle. The form and size of the shell-aperture correspond with, and indicate the size of, the foot. In the pectinibranchiate Mollusks, which are the chief fabricators of the beautiful turbinated shells of the conchological cabinet, the foot is attached to the anterior part of the body by a narrow base, as in Rostellaria (fig. 88, d), whence they have been termed by Lamarck Trachelypods.

The primitive muscular fibre is smooth in all Gastropods. The primitive fasciculi have often numerous nuclei scattered through them, and will illustrate Kölliker's beautiful discovery of the nature of the smooth organic fibre as a much elongated cell-wall. The cutaneous muscular layer consists of oblique longitudinal and transverse fibres, intimately united with the corium. Upon the ventral surface it becomes very thick, and forms a long disc called the "foot," marked d in all the figures of the Enephalia. The fibres of this part contract successively, so as to form wrinkles or transverse waves, following each other from behind forwards, whereby the disc glides over solid bodies or the surface of water. The circular foot of the limpet is used as an adhesive sucker. In some species it expands to a great breadth. In Siphonotus, Gasteropteron, and other Tectibranchs, it develops lateral swimming lobes, which are analogous to the fins of the Pteropods. In many Gastropods the foot is extended lengthwise, and more or less cleft transversely, as in Phorus (fig. 78) and Rostellaria (fig. 88). The anterior division d forms the chief creeping disc; the posterior one e supports the operculum f when this is developed. The posterior lobe of the foot in the inoperculate Harpa is said to separate spontaneously when the animal is irritated. In Atlanta (fig. 65, e) it is compressed, but supports the operculum f. It seems also to form the tail in the beautiful Carinaria (fig. 70, C), in which the middle part of the foot is reduced to a compressed fin-shaped lobe (fig. 70, B) supporting a small suctorial disc d. This Mollusk and its allies, hence called Heteropoda, swim on their back with their reduced locomotive organ upwards. In most Gastropods the tentacles, buccal mass, and penis, have their special retractor muscles. The opposite extreme of development of the foot is presented by some of the Neritacea. The coriaceous foot of

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1 Phil. Trans., 1785, p. 343. the burrowing Natica (fig. 80, a) is much developed anteriorly for perforating the sand, and can be reflected so as to protect the tentacle in that act. The sides d are produced for locomotion in water. The hind part of the foot h is expanded so as to cover the shell like a mantle-lobe.

In the living Cypraea (fig. 48) and Ovula (fig. 81), the mantle-lobes h, h are observed to be in almost constant tremulous motion; but the most vigorous muscular efforts in Gastropods are those of the foot, combined with the retractor-shell muscle. The strombs and scorpion-shells thereby progress by successive jumps. The active olives can turn over when laid on their back, and bury themselves in the sand as the tide retires. The periwinkle advances alternately the sides of its longitudinally-indented foot; Buccinum arcuaria defends itself by its dentated operculum.

At the grade of the Molluscous organization which the Gasteropoda have reached, their capabilities and spheres of action become more extended and diversified than in the Pteropoda and Acephala; some are terrestrial, some arboreal; whilst the more numerous aquatic species are endowed with power to attain, subdue, and devour organized matter, dead and living. The nervous system of the Gastropods is accordingly not only more complex and concentrated—not only subordinated to better developed masses in connection with organs of special sense and exploration,—but it offers greater variety in its general arrangement, and especially in the position of its ganglia, than in the Lamellibranchiate class; and with these modifications, considerable differences in the outward configuration of the body are associated. A few Gastropods, for example, are symmetrical, more or less flat, or depressed; others are compressed; the majority are contorted, and lose their symmetrical form in an oblique twist. There are other diversities of organic structure which more immediately affect the condition of the nervous system, for some species possess both eyes and tentacles; whilst others are blind and akerous.

The nervous system has a distinct fibrous neurilemma, often charged with pigment cells. The ganglion-cells are often pedunculated, and have usually a very large nucleus, composed of obscure granules, in the midst of which are two or four transparent nucleoli. Such cells form the sole contents of the neurilemmal canal at some parts of the nerves. (Lewes.) The central part of the system surrounds the oesophagus, and consists of different parts, of which that placed above the tube, and which usually includes two contiguous ganglia, is called the brain (fig. 70, u).

In the limpet (Patella) and bubble-shell (Bulla), we find that the cerebral ganglia, as in the Bivalves and Pteropods, are still distant from each other, and situated at the sides of the oesophagus, connected together by a nervous chord or commissure, which arches over that tube; from these ganglia two nerve-trunks proceed backward on either side; the median and superior pair pass along the sides of the oesophagus, converge, and meet below to form a pair of ganglia in close contact with one another, which supply the foot; these are evidently homologous with the bi-lobed pedal ganglion of the Mytilus. The lateral and inferior filaments pass downwards to join two widely-separated branchial or splanchnic ganglia, homologous with those situated on the posterior adductor in the Mytilus. There is, however, a considerable difference in the relative positions of the pedial and branchial ganglia in the limpet; the latter have advanced into close contiguity with the pedial ganglia, and are connected with them by the same transverse chords, which in Pecten and Mytilus serve merely to bring the branchial ganglia themselves into mutual communication. The position of the cerebral ganglia varies according to the degree of extensibility of the mouth and oesophagus. Thus in the snails (Helix) they are placed above the mouth; in Carocolla at the commencement of oesophagus; in the whelk (Buccinum), near the end of that tube; in the Purpura beyond the stomach.

As a general rule, we find that the superior ganglia give off tentacular, ocular, and buccal nerves; whilst the inferior masses are the centres of the muscular, respiratory, and visceral internunctiate chords. In the Carinaria (fig. 70), the superior or cerebral ganglia are marked u; they supply the eyes C, the tentacles b, the acoustic sacs u, and complex parts of the mouth; the buccal ganglia, situated above the glottidium a, and developed upon the buccal nerves, may well be deemed the centres for the reception of such degrees of smell and taste as the Mollusk may enjoy. We have here, therefore, indisputably the centres of the nervous system which are analogous to the olfactory, optic, and acoustic divisions of the brain of Vertebrata;—Who may say that they are not also the rudimental homologues of such? Upon either basis the part of the Mollusk "above" or "dorsal" of the digestive canal is thus defined.

With a difference so great in the disposition of the locomotive organs and muscular masses in the Vertebrate and the Mollusk, corresponding differences in the ordinary excito-motory nervous centres must be looked for. In the Carinaria they are situated at r, above the base of the compressed foot B; they are connected, as in Mytilus by long commissural chords with the cerebral ganglia, and by a short, thick commissure with each other; thus completing a wide and loose oesophageal ring. Nerves or commissural chords pass from both the cerebral and pedial ganglia to the splanchnic or branchial ganglion x; situated, like the pedial, below the alimentary canal, and supplying that canal, the liver, heart, branchiae, and genital glands.

In the spiral Pectinibranchiate Univalves, where the branchiae and their nerves are twisted to the left side, it is the left branchia which is atrophied, while the right one is of large size. The nerves are similarly affected; the left branchial one being filamentary, whilst the right is a large chord, and has the branchial ganglion developed upon it.

The principal oesophageal ganglionic circle, in most Gastropods, more closely surrounds the gullet than in Carinaria, and is defended by a thick membrane, which, in the large Tritons, assumes almost a cartilaginous hardness. A coloured pigment is not unfrequently found occupying a position analogous to that of the arachnoid, between the dense outer membrane and the ganglia. In the Limnea and Planorbis this pigment gives to the ganglia their orange or roseate hue.

Amongst all the observed diversity in the number, size, and position of the nervous masses of the Gastropods, certain ganglia are obviously homologous with those which have received determinate names in the Lamellibranchiate Mollusks. The branchial or splanchnic ganglia (figs. 18 and 80, x) receive impressions from, and transmit them to, the gills, heart, and other viscera; they communicate with the brain, and either through that centre, or by more direct connection with the pedial ganglia, associate the circulating and respiratory forces with all other parts of the body. In the Gastropods which have great expanse of soft mucous integument exposed,—as, e.g., Aplysia, Umbrella,—accessory small ganglia, supplying such integument, are developed, which are in more immediate connection with the splanchnic ganglia. The pedial ganglia are more commonly distinct than in the Bivalves; and the two divisions are, in some Gastropods, wide apart, in consequence of the great breadth of the foot; in most Gastropods the acoustic sacs receive, as in Bivalves, their nerves from the pedial ganglia. The cephalic ganglia assume the character of optic lobes, concurrently with the constancy and better development of the eyes. Even when the organs of vision are more than usually minute, or are wanting, these ganglia are always larger than in the Acephala, and more decidedly superior in position; they supply also the acoustic vesicles in many Gastropods, e.g., in the whole order of Nucleobranchiata, and in some Nudibranchiata. The cephalic ganglia, when separate, are united by a thicker communicating chord, and are larger, in proportion to the nerves given off from them, than in the Acephala.

Soft, lubricious, and irritable as is the exposed skin of Gastropods, it would seem to possess a very low degree of true sensibility. Baron Férisson affirms that he has seen a slug allow its skin to be bitten by others, and, in spite of large wounds thus produced, it has manifested no sign of pain. Species of Boilus, in confinement, will devour each other's branchia; which may be reproduced. The vascular inferior surface of the foot may take cognisance of the character of the surface over which it glides; but the special organs of the tactile sense are the tentacles or "horns" which project from the lateral and upper parts of the head. It is most probable that, with the exception of the cephalic ganglia, the functions of the nervous centres are limited to the automatic reception and reflection of stimuli, like those of Hartley's first class of vibrations in man, depending on nervous influence "which is detached down the motory fibres before reaching the brain."

The common snail offers a fair type of the digestive system of a Gastropod. In fig. 63, a represents the dentated horny jaw which arms the upper lip; b the large white salivary glands that spread upon the sides of the wide gullet; f is the stomach; g a caecal production which may represent the rudiment of a pancreas; u is the intestine, the part traversing the respiratory chamber, and commonly called rectum, laid open, terminating at i; i show the lobes of the liver. That typical bend of the gut, whereby its outlet is brought into communication with the respiratory and renal chamber is also illustrated in fig. 65, at o (Atlanta); and again in fig. 70, at no (Carinaria); the nearer approach to the straight course shown by the intestine of Doris or Firola (fig. 71, mo), is an exceptional departure from the Molluscous type, and is due to an equally exceptional position of the breathing organ.

Other well-marked modifications of the digestive apparatus will be noticed in connection with the species of Gasteropoda that may present them. The blood of the Gastropod is often opalescent, with a few colourless corpuscles or cells having an indistinct granular nucleus.

The auricle is divided in Fissurella and Chiton as in Haliotis. Both auricles, however, equally receive the oxygenated blood from the respiratory organ, as does the single auricle in Atlanta (fig. 65, z), in Helix (fig. 63, e), and in all the other Gastropods. The ventricle (fig. 63, p) propels the blood to the viscera and muscular system of the body; and the heart is situated on the right side of the back in the Pulmonata, most Tectibranchiata, and the dextral Pectinibranchiata. It is on the opposite side in Ancylys, Haliotis, and the sinistral Gastropods. It is to the left of the dorsal median line in Carinaria, and near the hinder end of the body in Firola. The heart has a distinct pericardium in all Gastropods, save the Apneausta, where it is at least not clearly defined. The aorta, continued from the apex of the ventricle, divides into two principal branches in most of the Gastropods. The auriculo-ventricular aperture is usually defended by two semilunar folds (fig. 63, between o and p). The aorta at its commencement is frequently strengthened and enlarged by a muscular layer similar to the bullas arteriosus in fishes, and which, in the Aplysia, is continued beyond the origins of the primary branches of the aorta. The ramifications of the aorta, as in crustaceans and insects, are sooner or later lost in veins which expand to form sinuses, occurring in the lacune of the viscera and other organs of the body. The anterior aorta terminates,—e.g., in Patella, Triton, Haliotis,—in a large lacunar sinus, containing the brain, the salivary glands, the esophagus, and retracted tongue. The resumption of the normal vascular character by the venous system is more or less sudden, and is best exemplified near the respiratory organ (fig. 63, nu), upon which such venous trunk ramifies like an artery, without any interposed branchial or pulmonic heart. The large venous cava of the Aplysia are perforated by minute apertures, communicating with the great sinus that lines the cavity of the abdomen; and the exterior of these veins is provided with decussating muscular fibres, which probably regulate the diameter of such communications.

The diffused condition of the vascular system most prevails in those Gastropods in which the respiratory organs are least developed—e.g., the Apneausta. In the rest of the class the general modifications of the respiratory organs are indicated by the characters of the orders already defined. In the terrestrial Gastropods the breathing organ has the form of the simple undivided vascular sac (fig. 62, nu), like the lung in the lowest air-breathing vertebrate animals. Its orifice m is on the right side near the head. The forms of the aquatic-breathing organ are as various as its position.

In most of the Nudibranchiate species the gills are tufted and ramified, as in the higher Annelides. They are penniform in the Haliotis, and pectinated in all the diocious Gastropods (fig. 77, i), as the name of their order indicates. In these they never exceed two in number, which are of unequal size; and the branchial chamber is usually prolonged into a siphon. In a few genera of amphibious Gastropods a pulmonary sac is combined with branchial organs; but in some of these,—as, e.g., Amphibola (Amphullacea) australis,—the branchia are reduced to a mere gland. The branchial surface is ciliated in all the Gastropods, as is also the exterior surface of the body in the small fresh-water species.

As a compensation for the absence of gills, some Apneausta (Acteon, e.g.) have an aquiferous system, consisting of a reservoir filled with water, behind the heart, from which branched canals pass off in all directions, one of which, according to Vogt, opens on the right side behind the vent. Conspicuous aquiferous pores are situated at the centre of the foot in Cypraea, Conus, and Ancillaria; and at its margin in Haliotis, Doris, and Aplysia. Della Chiage has described a similar system in many of the higher organized Gastropods. Besides the large and well-developed hepatic and salivary glands which are associated with the alimentary canal, certain fucivorous Gastropods present the simplest rudimental condition of the pancreas.

The follicular renal gland, sac, or surface, for the urinary excretion, communicates by the canal called its duct with the external surface near the anus, and by an opposite opening with the pericardial cavity. The sac has been seen to contract rhythmically in the transparent Pteropods and Heteropods, and may perform the same movements in other Acephala. Water seems to be freely admitted to this glandular cavity. In some Gastropods the duct dilates to form a small receptacle. A group of follicular glands, sometimes imbedded in a distinct glandular sac, is present in many species for the elimination of some peculiar and characteristic colour. The yellow liquid of the Bulle, and the famous purple secretion of the Purpura, are products of saccular modifications of this follicular gland, which is situated between the heart and liver. Numerous simple and scattered follicular glands lubricate the mantle with its characteristic mucus in all the Gastropods. In several

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1 Hartley, On Man, &c., 8vo, 1749, vol. I., p. 97. 2 Pouchet, Recherches sur l'Anatomie et Physiologie des Mollusques, 4to, 1842; Milne Edwards, Observations sur la Circulation chez les Mollusques, Annales des Sciences Naturelles, tom. viii., 1847. terrestrial species the median line of the foot is occupied by a straight canal, lined with ciliated epithelium, which ends in a large orifice beneath the mouth. On each side of this canal are rows of follicles, which secrete a granular mucus. A follicle excreting a similar mucus opens on the extremity of each dorsal or lateral lobe in the Apneusta which possess those appendages. "Some of the Gastropoda can suspend themselves by glutinous threads, like Littorina and Lissota parva, which anchor themselves to sea-weeds (Gray), and Cerithidea, which frequently leaves its proper element, and is found hanging in the air. (Adams.) A West India land-snail (Cyclostoma suspensum) also suspends itself. (Guilding.) The origin of these threads has not been explained; but some of the Limacidae lower themselves to the ground by a thread, which is not secreted by any particular gland, but derived from the exudation over the general surface of the body." (Lister; D'Orbigny.)

Some peculiar scent-follicles must exist in the garlick-snail (Helix alliaria). A species of slug, called Phosphorax, is slightly luminous.

Gastropods have the power of repairing injuries and of reproducing lost parts to a considerable extent. New tentacula soon grow to replace those which may have been amputated. When they support eyes, as in the snail, the organs of vision are reproduced also. The mouth, with the horny jaw, has grown again in this Gastropod; and when the snail has been decapitated, but with the oesophageal ganglions left behind, the head has been restored.

**GASTEROPODA MONOECIA.**

**Order I.—APNEUSTA.**

No distinct respiratory organs. No shell (in the adult state).

Certain soft-bodied, subelongate, subdepressed, vermiform, aquatic animals, with a soft mucous ciliated integument, devoid of heart and respiratory organs, have been placed in the Articulate province of the animal kingdom, and therein have been associated with the Annelides, or with the Entozoa, or have been kept apart in an intermediate class, under the name of Turbellaria. In some of these worms (fig. 1) the alimentary canal, commencing by an evertible proboscis, sends off numerous branched blind-tubes, which radiate into the surrounding parenchyma. The oesophagus is surrounded by a nervous collar, developing two oesophageal ganglia, from which the nerves are sent off to the body. Some species have two cephalic tentacles (Planaria tentaculata; Physanoxoon, Grube). The ciliated skin appears to perform the function of respiration. No heart has been detected. The male and female sexual organs are combined in the same individual.

Similarly-shaped small marine animals, with a ciliated soft integument, equally devoid of heart and branchiae, with the nervous system, the digestive and the generative organs, organized according to the same type as in the above-cited Turbellaria, have been placed in the Molluscous province. Such, e.g., are the species of Rhodope and Acteon. The transition from these abranchiate and acardiac worm-like animals to the Eolidians, which have a heart, but are without gills,—Calliopea, e.g. (fig. 2),—and from these to the true Nudibranchiates,—Doris and Tritonia,—is so close, that in drawing the line of demarcation in the series progressing from the Trematode Entozoa, through the Planaria upwards, we define the Molluscous boundary as arbitrarily as in the series progressing from the Bryozoa, through the Tunica, to the same great province of the animal kingdom.

If the diffused and ramified type of the alimentary canal were taken as a character of Turbellaria, as contradistinguished from these Mollusca, we should find it in some,—e.g., Eolis and Calliopea,—associated with a distinct heart; and in others,—e.g., Elysia,—associated also with a respiratory organ of the type of that of the pulmonated Mollusks. If we were to require definitely-developed respiratory organs as a title to admission into the Molluscous province, we must exclude true Gastropods, like Glancus and Calliopea, which are nevertheless too closely allied to Scylirea and Doris to be placed in a distinct primary division of animals from them.

For those naked Gastropodous Mollusks in which the ramifications of the alimentary canal take the place and discharge the functions of a liver, and in which, although there may be productions of the skin simulating the more localized branchial processes in higher Gastropods, those productions receive gastro-hepatic caeca, not branchial vessels—for the reception of such low-organized Mollusks—an order or sub-order is requisite, for which the term Apneusta ("gill-less") may be retained. The order may be characterized as one of "Naked Gastropods, hermaphrodite, marine, without distinct respiratory organs; alimentary canal with gastro-hepatic ramifications; anus dextral. The species often swim on the back, with the foot supine."

**Genus Rhodope**, Kölliker.—Body naked, smooth, sub-depressed, without tentacles; organs of circulation unknown. The comparatively simple form, of the peculiar digestive appendages of this species is an instructive one. Had it first been observed, instead of the more complex and ramified condition of the same parts, as in Calliopea, it most probably would not have been mistaken for a "gastrovascular apparatus," or have given rise to the ideas originally broached and generalized under the term "phleboterisme." The fry have no deciduous shell; they swim, like the Turbellariae, by means of superficial vibratile cilia.

Sp. Rhodope verani.—About 10 lines in length. Hab. Mediterranean.

**Genera Lissosoma, Quatrefoiges; Flabellina, Ed.; Phyllirhoe, Péron.**

Sp. Phyllirhoe Bucephalum, Lam.—Body oval, elongate, compressed, pelucid, without a trace of "foot;" coloured, with brown specks, head distinct, with two long, subulate, twisted tentacles; tail truncate, rounded; a pair of horny jaws; glottidium, with 3—0—3 teeth on the tongue-strap. Size of the animal varies from 8 lines to an inch. Hab. common in the Atlantic.

Six species of Phyllirhoe have been named and characterized; but the variability in the form of the soft contractile body, and in the superficial punctation and deeper-seated (visceral) coloration of the body, is such as to throw considerable doubt upon the constancy of the differences of colour which have been interpreted as specific.

**Genus Fucola**, Quoy.

Sp. Fucola rubra, Quoy and Gaim. (Voyage de l'Astrolabe, pl. 24, figs. 21, 22).

**Genus Acteon**, Oken (Elysia, Risso).—Body subcylindrical, limaciform; head bordered by a depressed expansion, pointed behind; two sublavate non-retractile tentacles, with eye-specks behind them; vent, in some lateral and dextral, in others subdorsal; glottidium, with uniserial dental plate.

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1 Woodward, Manual of Mollusca, p. 103. 2 Rhodope, Nuovo Genere di Gastropodi; Kölliker, in Giornale dell' I. R. Instituto Lombardo, Milano, 1847. 3 Kölliker, loc. cit., pl. I., fig. 1. 4 Sur l'existence d'un appareil gastro-vasculaire chez la Calliope de Risso (E) Mollusque de la famille des Eolidiens, par M. H. Milne Edwards, Annales des Sciences Naturelles, 2e série, tom. xviii., p. 330, 1842. It must be remarked, however, that the original discovery of the gastro-hepatic canals in the Eolidians (Prof. Lowén), entertained from the beginning correct notions of their nature and functions. Sp. *Acteon corrugatus*, Forbes and Hanley (*Brit. Moll.*, t. ccc., fig. 5). Sp. *Acteon sinister*, Quatrefages (*Ann. des Sciences Nat.*, 1844, pl. 3, fig. 4; *A. viridis*, ib., pl. 3, fig. 2; *A. elegans*, ib., pl. 3, fig. 3).

*Genus Limapontia*, Johnstone.—Body oblong, depressed anteriorly, raised and rounded behind; head arched and keeled on the sides; eyes behind the carinae; no tentacles; vent dorsal, sub-posterior.

Sp. *Limapontia nigra*, Forbes and Hanley (*Brit. Moll.*, t. ccc., fig. 4).

*Genus Calliopsea*, D'Orb.—Body oblong, subdepressed; head forming a kind of round swelling circumscribing the mouth, with only two (posterior) tentacles, bearing eyespots at their base; the foot slightly dilated anteriorly.

Sp. *Calliopsea Souleyetii* (fig. 57)—In this species the mouth is subterminal; the gottidium large, slightly protractile, and bearing a uniserial imbricated plate, with many denticles in each transverse row of the series. A slender, elongated, salivary follicle opens on each side of the pharynx. The oesophagus is very short, being defined simply by the nervous collar, beyond which it dilates into a small ingluvies, and afterwards into the stomach, from the termination of which the short intestine and the gastro-hepatic canals diverge. The latter are four in number, are diverging, in two lateral pairs (one forwards, the other backwards), and sending off the branched gastro-hepatic follicles from their outer side. The heart is situated between the two diverging posterior gastro-hepatic canals; the wide auricle receives the blood from the diffused sinuses which have exposed it to the respiratory influence acting through the delicate integument. The aorta divides into two principal branches. The ovispermal masses are divided into separate rounded bodies with their several canals, which unite into a trunk traversing an ovoid glandular enlargement; beyond this the oviduct separates from the sperm-duct, the former terminating in a long convoluted uterus with glandular walls, and the latter terminating in a long, tubular, evectible penis.

*Genus Cenia* (Syn. *Ictis*), Alder and Hancock.

Sp. *Cenia Cockii*, Forbes and Hanley (*Brit. Moll.*, t. ccc., fig. 6).

*Genus Pelta*, Quatrefages.

Sp. *Pelta coronata*, Quatrefages (*Annales des Sciences Nat.*, 1844, pl. 3, fig. 6).

*Genus Glaucus*, Forster.—Body elongate, slender, posteriorly filiform; foot linear, channelled; tentacles 4, conical; jaws horny; teeth in single series, arched and pectinated; gastro-hepatic caeca prolonged into three pairs of lateral, digitate, or palmate tegumentary sacs. Colour blue; length between 1 and 2 inches. Hab. Atlantic Ocean, between the tropics.

Mr Bennett, who captured some of these beautiful little pelagic Mollusks in Lat. 2° 26' N., Long. 19° 51' W., has recorded some interesting observations on their living colours, movements, and food, in the *Proceedings of the Zoological Society*, 1836, p. 113. The gastro-biliary and tegumentary productions, which some have called "gills," he terms "fins." As they floated upon the surface of the water (in the glass), the light silvery blue of the lateral parts contrasted with the deeper blue of the upper surface. This surface seemed to be most sensitive or irritable. When touched the animal coils itself up, then dashes out into the linear form; again coils itself up, and so remains for a short period, apparently exhausted by its efforts. "But on the cessation of the irritating cause, the animal quietly resumed its original position, perhaps dropping one or two of its weird fins, according as its own sensations of ease or comfort might dictate.

"When nothing irritated this tender Mollusk, it would remain tranquilly floating upon the surface of the water, with scarcely any movement but that which proceeded from the undulating movements of the digitated extremities of the fins, as well as an occasional slight twisting motion of the same organs.

"I felt much interest in the beautiful display of a circulating fluid on the dorsal surface of these animals, which was afforded me by the assistance of a microscope. Through the semi-transparent membrane of the back a fluid could be readily perceived close to the surface, evidently flowing in two directions, one taking a course downwards, and the other returning upwards; but I was unable to distinguish two distinct vessels for these separate actions.

"These animals seemed to be very torpid in their movements, although sometimes, when floating upon the water, they would be seen busily engaged in moving their fins about; but those actions were soon suspended, and their fins were suffered to hang lazily down, as if fatigued with the short exertion, which did not move them one inch about the glass of water; and even when the little indolent creatures did take the trouble to move themselves from one side of the glass to the other, it was effected by a tardy motion, stirring themselves first with one fin and then with the other, according as circumstances might require.

"I placed some small specimens of *Porpita* in the glass of water containing the *Glaucus*, to observe if they would attack them; for some time one of the *Glaucus* was close to a *Porpita*, and was even annoyed by the tentacula of the latter touching its back; yet the *Glaucus* bore this, although with the usual characters of impatience, yet without attempting to attack it. At last it seized the *Porpita* between its jaws; and by aid of a powerful lens, an excellent opportunity was afforded me of closely watching the devouring process, which was effected by an apparently sucking motion; and at this time all the digitated processes of the fins were floating about, as at other times when the animal was at rest. But I did not observe in one single instance that they were of any use to the animal, either to aid in the capture or to securely hold their prey when in the act of being devoured; for the animal seems to depend merely upon the mouth in capturing its prey; as in this and other instances, which I had opportunities of observing, they seized their prey instantly with the mouth and held it by that power alone, whilst by a kind of sucking motion the prey was devoured.

The digitations may therefore only be regarded as appendages to the fins to aid the animal perhaps in the direction of its movements, as it was observed that they turned and twisted them about during the progressive motion (that is, when this tardy animal is pleased to progress, which appeared to me very rarely to meet with its inclination), as if in some way or other to direct the movements of the animal.

"The *Glaucus*, after eating the tentacles and nearly the whole of the soft under surface of its prey, left the horny portion, and remained tranquilly reposing upon the surface of the water after its meal, the only motion visible in the animal being the playing of the digits of its fins. The mutilated remains of the *Porpita* sank to the bottom of the glass.

"Soon after another *Glaucus* began a devouring attack upon another *Porpita* which had been placed in the glass, eating a little of it, and then ceasing after a short meal, occasionally renewing the attack at short intervals. On examining the Porpita, which had been partially devoured by the ravenous Glauces; I found the disc had been cleared of the tentacles and other soft parts; a small part of the fleshy portion only remaining upon the disc. Only one part of the horny disc exhibited any injury, and that appeared to be the place where the animal was first grasped by the Glauces.

"When any of these animals came in contact with another in the glass, they did not display any annoyance, or coil themselves up, nor did they evince any savage propensities one towards the other; and they would often float about, having their digitated processes in contact one with the other, without exhibiting any signs of annoyance. Even when placed or pushed one against the other they did not manifest any irritation, but remained undisturbed, as in their usual moments of quiet repose.

"These animals soon perished. I could not preserve them for any length of time in the glass of sea-water, although the water was changed as often as it was thought necessary; the digitated processes of the fins were observed to shrink up on the death of the animal, and the process of decomposition rapidly took place, the whole body becoming a shapeless mass, having a bluish colour of deadly hue for a short period, and then became of a blackish or brownish-black colour. I have seldom seen a gelatinous animal which appeared so firm whilst in the water, that proved so speedily to decompose when removed from it. Even the beautiful purple of the back, the silvery or enamel of the abdomen, and the silvery blue of the sides, all speedily vanish, indeed instantly disappear, upon the death of the animal, as if it had been washed off; the expansive, delicate, and beautiful fins and digitated processes are no longer seen; they shrank up to nothing."

ORDER II.—NUDIBRANCHIATA, Cuvier.

Branchiae extending more or less freely from various parts of the body.

Genera Scylla, Doris, Thetis, Polyera.

Genus SCYLLA, Linnæus.—Body elongated compressed; foot long, narrow, furrowed longitudinally; head furnished with two retractile tentacles; back with two expansions or membranous and flexible wing-like lobes on each side; branchiae penicillate, composed of filaments, scattered over back, especially crowded in the wings.

Sp. Scylla pelagica, L., Cuv. (Ann. du Mus. vi., pp. 416, 417, pl. 61, figs. 1–7; Mollusc. Mem.; Blainv. Malac., pl. 46, fig. 5).—This species attaches itself by its furrowed foot to Fucus natans, and is found in the Atlantic Ocean. The glottidium has a triserial rasp, with 24–1–24 teeth; the gizzard is armed with chitinous trenchant plates.

Genus TETHYS, L.—Body somewhat long, depressed, furnished anteriorly with a broad, funnel-shaped, fimbriated disc, behind which it contracts into a sort of neck; mouth proboscidean retractile. Two conical tentacles at the base of the disc retractile into broad cup-shaped sheaths. Apertures of generation and vent at the right side in the anterior part of body. Two rows of branchiae at the sides of back; cirrose pectinate branchiae alternating with smaller bundles.

Sp. Tethys teprina, L., Rondelet, Pise., p. 526; teria teprina marini Species; Cuvier, Ann. du Mus. xii., pp. 259–270, pl. 24, Mem. sur les Moll. vii.; Blainv., Molacol., pl. 46 bis, fig. 9).—This species sometimes attains a foot in length. Hab. the Mediterranean.

Genus DORIS, L., Cuvier.—Body oval, flat, or gibbous above, with abdomen flat, covered by a loose membrane, and plicato-marginate. Vent posterior, dorsal, in the mid line of body, surrounded by branched or plumbed branchiae disposed in a circle. Apertures of generation at the right side. Two dorsal tentacles retractile within tubes, annulate with transverse lamellae; two quasi-tentacular productions from near the mouth. Mouth armed with two horny plates (fig. 29, a), united near the front, and having two projecting points; glottidium (fig. 29, d) having lingual teeth numerous, central small, laterals similar, hooked, and sometimes serrated.

Genus TRITONIA, Cuvier.—Body elongate, subtetragonal or compressed, anteriorly rotundate, posteriorly acuminate; apertures of generation and of rectum at the right side, with vent situated behind the genital orifice. Two tentacles with branched filaments retractile into a sheath. Circular velum, tuberculated or digitated, in front of mouth. Two lateral jaws, acute, with margin denticulate. Branchiae arborecent at the sides of back. Glottidium with triserial tooth-strap, one central, and numerous lateral teeth in a row.

ORDER III.—INFEROBRANCHIATA, Cuvier.

Branchiae at the lower part of the sides of the body between the foot and mantle. The heart lies in these Molusks in the middle of the body on the dorsal surface, and receives the blood from the gills placed on each side. Both "Prosobranchiates" and "Opisthobranchiates" are contained in this very natural group.

Genus PHYLLIDIA, Cuvier.—Body oblong, with a tuberculated mantle; head with four tentacles, the two dorsal retractile within a cavity; the other two labial; anus in posterior and middle part of back. (Cuvier, Ann. du Mus., v., pp. 266–276, pl. 18; Mollusc. Mem., viii.)

Sp. Phyllidia trilineata, Cuvier; Phyll. varicosa, Lam., Cuvier, l. c., figs. 1–6; Blainv. Malac., pl. 47, fig. 1).—Hab. Indian and Red Seas.

Genus Pleurophyllidia, Meckel. (Diphyllidia, Cuvier; Linguella, Blainv.).—Body oblong, with ample mantle, acuminate posteriorly; head with two tentacles placed towards the back at the anterior margin of mantle; frontal veil with angle produced on each side; gills limited to the hinder two-thirds of the body; vent on right side, behind genital foramen.

Sp. Pleurophyllidia lineata (Diphyll. lineata, Otto, Nov. Act. Acad. Ces. Nat. Cur. xviii.; Delle Chiaje, Memorie i., p. 128, tab. 10, figs. 12–20; Meckel, Archie, f. d. Physiol. viii. 1823, tar. ii., figs. 1–7, s. 190–207).—Hab. the Mediterranean, and, according to Lovén, the North Sea also.

ORDER IV.—TECTIBRANCHIATA, Cuvier.

Branchiae resembling pinnatifid leaves, restricted to one side, and covered by the mantle and a small shell, which is sometimes exposed. Most are phytophagous; all have a complex gizzard.

Genus APLYSIA, L., Gmel.—Body oblong, limaciform, mostly margined by a broad velum reflected from the sides of the foot over the back; two contractile tentacles, conical, sulcated, in upper part of the head; two productions of the velum surrounding the mouth, forming as it were, a second pair of inferior tentacles; eyes sessile, in front of base of superior tentacles; branchiae dorsal, covered by a production of mantle, including a flat membranoso-corneous or calcareous shell, and folded posteriorly so as to form an excretory siphon.

Sp. Aplysia depilans, L.—The tooth-strap of Aplysia depilans is broad, short, brown. Teeth brown, many on each diverging cross series; central tooth truncate, triangular, dilated beneath, with an arched base, and tridentate apex; lateral teeth 12–12, in an oblique line, each tooth arched, with the tip recurved, with three rounded lobes on the lower edge; the crown of the anterior teeth worn, so as to leave the reflexed and lobed part. The gullet dilates into a large crop; the thick coats of the gizzard are beset with firm horny processes, some in the form of hooks, others of rhomboid crushing plates. Near the pylorus there is a long pancreatic caecum.

The *Aplysia depilans*, or "sea-hare" of the Mediterranean, sometimes attains a foot in length. (For the other species of this singular genus, see Sander Rang, *Hist. Nat. des Aplysies*, Paris, 1828, folio.)

"The sea-hares are mixed feeders, living chiefly on seaweed, but also devouring animal substances; they inhabit the laminarian zone, and oviposit amongst seaweed in spring, at which time they are frequently gregarious. (Forbes.) They are perfectly harmless animals, and may be handled with impunity. When molested they discharge a violet fluid from the edge of the internal surface of the mantle, which does not injure the skin, has but a faint smell, and changes to wine-red. (Goodir.) In old times they were objects of superstitious dread, on account of their grotesque forms, and the imaginary properties of their fluid, which was held to be poisonous, and to produce indelible stains." (Woodward.)

**Genus Dolabella**, Lam.—Operculum of branchiae towards the posterior part of back, including a calcareous shell. Body mostly truncated posteriorly with an orbicular declining area.

Sp. *Aplysia Rumphi*, Rang (*Dolabella Rumphi*, Cuvier; Rumph, *Amb. Rariteitk.*, tab. x., fig. 5, tab. xi., fig. n the shell); Cuvier, *Ann. du Mus.* v. p. 437 and foll., pl. 39, figs. 1–4, *Moll. Mém.* 12.

**Genus Siphonotus**, Adams.—Body elongated; gills covered by the mantle and shell; foot with the sides dilated into swimming lobes; respiratory orifice prolonged into a siphon. Shell nearly membranaceous.

Sp. *Siphonotus geographicus*, Adams.—Whitish-brown, covered with minute dark specks, and large, irregular, green, reticulated patches, margined with opaque white; under surface of foot of a bright yellow, left side of foot with a projecting lobe, which overlaps that of the opposite side; siphon of the mantle prolonged into a tapering, subcylindrical tube. Hab, Java Sea, among masses of floating sea-weeds.

**Genus Umbrella**, Chemnitz.—Body chiefly composed of a large, thick, tuberculated foot, deeply notched anteriorly; mouth (fig. 58, a) proboscisiform, retractile into the pedal notch, overhung by a small velum; tentacles two, with plicated cavities at their base; eyes sessile between the tentacles; genital orifice in front of the tentacles; excretory orifice posterior, tubular. Shell orbicular, almost flat, with a subcentral low apex.

Sp. *Umbrella mediterranea* (fig. 58).—The tooth-strap

of *Umbrella mediterranea* is broad, longitudinally plaited; teeth in numerous oblique series, each containing numerous small, closely pressed, compressed, sharp-edged, transparent teeth.

**Genus Bulla**, Lam.—Velum of head large, separated from the mantle by a transverse furrow (fig. 59, m), which invests the shell z'. Vent, orifice of generation, branchiae, and heart placed at the right side. Foot broad, shorter than body, with the lateral margins l produced, but not enveloping. Tooth-strap with two or four series of sickle-shaped teeth. Shell thin, convolute, with aperture large.

Sp. *Bulla aperta*, Lam. (fig. 59).—The tooth-strap of this species is thin and narrow, with brownish transparent teeth; two in each transverse row, close together at the base, long, slender, compressed, arched, nearly in half a circle, with the inner edge finely denticulated. The gizzard is armed with three long, partly horny, partly shelly plates. The eggs are in a single series, in long spiral gelatinous filamentary capsules. The fry are furnished with ciliated vibratile swimming lobes, and have a spiral, operculated shell.

**Genus Scaphander**, Montf.—In this genus the shell is oblong, convolute, spirally striated, with the aperture much expanded in front; the spire concealed.

Sp. *Scaphander lignarius* (*Bulla lignaria*, L.).—The teeth are in two longitudinal series, claw-shaped; crenulate on the convex margin near the point, with a crest on the same margin near the base. The gizzard is provided with two large triangular calcareous plates, united together by strong transverse muscular fibres attached to their circumference, except at the upper part, where a third small oblong plate is interposed between the two lateral ones.

The *Scaphander lignarius* feeds upon the *Dentalium entale*; five of these have been found in the gizzard, with the shells partly crushed and digested.

**Genus Tornatella**, Lam.—Body ovate, white; head broad, notched in front, with a pair of eye-specks (fig. 60, o), and two retroverted tentaculiform lobes h; the foot has two small anterior productions b; slightly developed lateral lobes support the shell, and an opercular appendage f.

Sp. *Tornatella tornatilis*, Lam. (fig. 60).—Hab. British seas, in deep water.

**Genus Bulla**, Lam. (fig. 61).—Head in the form of a large disc a, truncate, and often mesially notched in front; produced above into lobes b; laminated beneath, and sometimes with lateral appendages b; two subcentral eye-specks; side lobes of the foot large. Shell ventricose, convolute, partially exposed; aperture long, lip sharp.

*Bulla vexillum*, Chemnitz (fig. 61) after Adams.—This most beautiful species was captured in sea-weed, in about a fathom water, near Ambolan, Mindoro, by Mr Adams. The animal was of a delicate pink colour, with the cephalic disc and lobes, and the foot, edged with white and red. The foot, when not expanded for swimming, is folded upon the shell. The inner margin of the mantle forms a thick fleshy lobe, which partially fills the hind part of the shell aperture; the outer margin lines the outer lip.

Fig. 62 gives the character of the dentition in the Bullidae, the central teeth being small and few, as in Akera bullata, or wanting; the laterals single, slender, and recurved.

According to Cuvier, the cephalic lobes b of the Bullidae, represent the fused oral and dorsal tentacles. In the common "bubble-shell" (Bulla Hydatina) they consist of a central stem bearing numerous lateral plates, and are conjectured by Mr Hancock to serve as olfactory organs.

Order V.—PULMONATA, Cuvier.

Part of the mantle-cavity forming a vascular air-sac or lung.

This well-marked order of Mollusks is treated at length in the illustrated work of Baron Féussac, Histoire Natur. des Mollusques terrestres et fluviatiles, Paris, of which the first part appeared in 1819. M. Deshayes, after the death of Féussac, began the continuation, which was completed in 1831.

Most Pulmonata are terrestrial; those which are aquatic rise to the surface of the water to breathe. A few are naked; most are testaceous; and the latter are distinguished as having, or wanting, the operculum. The inoperculated Pulmonates have a tongue-strap, paved with rows of very numerous similar teeth, with broad bases; the operculated Pulmonates have a tongue-strap with arched rows of seven teeth, the median tooth differing from the three on each side.

Section I.—INOPERCULATA, Féussac.

Family I.—HELICID.E.

(Snails.)

Shell external, usually well developed, closed by an epiphragm during hibernation.

Genera Helix, Vitrea, Succinea, Bulinus, Achatina, Papua, Clausilia.

The common garden-snail (Helix nemoralis, L.) exemplifies the present family. The species selected by Bruguières from the wide Linnaean Helix to form his more restricted genus, have since been much subdivided by Conchologists. The following indicate some of the leading modifications of the shell:

† Aperture turned upwards, denticulate on each side.

Genus ANOSTOMA, Fischer, Lam.

Sp. Helix ringens, L. (D'Argen. Conchyl., pl. 28, figs. 13, 14; Blainv. Malac., pl. 39, fig. 4.)—Hab. East Indies.

Aperture inclined downwards; shell at the periphery carinate or subcarinate, mostly depressed. Labrum often reflected, sometimes dentate.

Genus CAROCOLLA, Lam.

Sp. Helix Lapicida, L. (Pfeiffer, Schn. i., tab. ii., fig. 26); Helix Carocolla, L. (D'Argen. Conch., tab. 8, fig. D; Guérin, Iconogr., pl. 6, fig. 1).—Hab. East Indies, &c.

Aperture inclined downwards. Shell rounded at the margin, most frequently subglobose.

Genus HELIX, Lam.

Sp. Helix Pomatia, L. (Sturm, Deutschl. Fauna, vi., Heft i.; Pfeiffer, Schn. i., tab. ii., fig. 9; Cuv., R. Anim., ed. 2, t. iii., p. 40, Moll., pl. 21). The Vineyard-Snail, le Grand Escargot.—The name Pomatia, from Πάμα, cover (Cochlea opercularis), was given to this animal on account of the closure of the aperture of the shell in winter that occurs in this and other species. In autumn the animal retracts itself within the shell, and then a false operculum (epiphragma) is secreted, which is pushed off in spring, when the snail again creeps out of its house. During the winter these snails in our temperate climate "hibernate," i.e., take no food, but lie torpid; in like manner, in tropical regions the species of this genus in the hot and dry months "activate," or fall into a similar state of torpidity.

The anatomy of this large species of snail has been admirably described by Cuvier, from whose monograph the preceding figure (fig. 63) is taken. The parts relating to the digestive, circulating, and respiratory systems have already been pointed out. The modification of the singularly-combined male and female apparatus, in this most highly developed of the monoecious Gastropods, calls for a more detailed notice.

The complexity and bulk of the combined organs in the vineyard-snail are truly extraordinary. The essential organs, testis and ovarium, are associated together in the form of a small compact gland (fig. 63, q), composed of many parallel cæca imbedded in the substance of a lobe of the liver i, and occupying the apex of the shell. Each cæcum consists of an external layer, producing ova, and an internal sac, folded in the first, producing semen.1 The walls of

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1 Meckel, II., Ueber den Geschlechtsapparat einiger hermaphroditer Thiere, in Müller's Archiv für Physiologie, 1844, p. 376. these invaginated sacs are usually in contact, but become separated at the points where the ova push the ovarian sac outwards, and the sperm-cells the testicular one inwards. The common ducts from those series of combined sacs are also invaginated, the oviduct being external, the sperm-duct internal, and usually undulated. The testicular ceca and sperm-duct are lined with ciliated epithelium; this is not present in the ovarian sacs.

Both invaginated tubes enter the albuminiparous sac s, and separate where they quit that part. There is a dilatation or sperm-reservoir where the sperm-duct quits the base of the albuminiparous sac, and this latter may be regarded as a more special dilatation of the oviduct, with follicular walls. The sperm-duct, enlarged and with more glandular walls u, now proceeds, with many short folds parallel with the uterus, to the base of the penis. This is a long and slender organ v, usually retracted and concealed within the visceral cavity, but, like the finger of a glove, capable of being everted and protruded externally. The so-called "uterus" t is a long canal, with transversely-plicated glandular walls, terminated by a vagina opening into the common genital vestibule, the external orifice of which is near the mouth of the respiratory sac, on the right side of the head. With the vagina there communicates the duct z of a small pyriform vesicle w, which is a sperm-reservoir for the fecundating element of another individual received in coitus. A small cecum is developed from the duct in Helix pomatia, and a very long one in Helix arbustorum; the duct is short and simple in the slug (Limax). The genital vestibule receives the terminal outlets of two groups of branched ceca x, x, or "multifid vesicles," the function of which is unknown. But the complexity of the generative apparatus does not end here; the small is provided with a pyriform muscular sac y, the aperture of which terminates close to the generative outlet. The expanded base or head of a slender conical calcareous style or dart is attached to the fundus of the sac: its sharp apex extends close to the orifice, and by the contraction of the sac it can be protruded outwards. With it the snails pierce each other's skin; and the function of this curious organ would seem to be to cause a preliminary excitement to the reciprocal union of the two androgynous individuals. The dart, like the epiphragm, is annually reproduced.

Of the rare Helix rittata (fig. 64), which inhabits Balambangan, Borneo, Mr Adams, its discoverer, remarks, that


the animal is of "a delicate subttransparent pinkish colour, the free lobes of the mantle moveable, and often extended from the fore part of the shell; eye-peduncles long, the truncatures for the eyes very broad, tentacles rather long and elavate; foot compressed, finely crossed with oblique lines, and margined inferiorly, the end with a large hollow muciparous follicle, ending below in a sharp, moveable, rather recurved process."

"This beautiful and singular species lives among the foliage of the low trees, about which it crawls with surprising rapidity, reminding one of the movements of the Vitrina more than those of the Helicidae." (Zool.Samarang, p.60.)

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**Family II.—Limacidæ.**

(Slugs.)

Shell rudimental, internal.

Genera—Limax, Arion, Parmacella, Testacella.

Genus Limax, Lam. (Ground-Slugs, Post-Slugs).—Body oblong, naked, convex above, furnished anteriorly with a coriaceous, subrugose mantle, and below with a longitudinal flat disc or foot, which is not distinct from the body; branchial cavity under the shield or small mantle, in the anterior part of back; respiratory orifice and vent placed in the right side; generative orifice beneath the right tentacles. The mantle in some contains a calcareous grit, in others a small shell; when alarmed, they withdraw the head beneath the mantle. At the hinder end of the body is a small aperture, from which adhesive mucous threads proceed. The slugs often climb trees in quest of the decaying vegetable matter on which they feed, and they lower themselves to the ground by means of those threads.

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**Family III.—Oncidiadæ, Woodward.**

No Shell.

Genera—Oncidium, Vaginulus, Peronia, Oncidoris.

The slugs (Limaceæ) and land-soles (Arions) of temperate climates are represented in the East by the Oncidium, Veronicella, and Peronia, as they are in the Western Hemisphere by the Vaginulus. The Veronicella lives upon the trees in the forests, and is active after showers; the Oncidia live on aquatic plants in marshes and ditches; while Peronia, like Oncidoris, lives among the stones on beaches, but, unlike the latter genus, above high-water mark, a little beyond the influence of the tide.

One species (Oncidoris celtica) is found on the coast of Cornwall, congregated in little groups, about a foot or two from the surface of the sea, where the waves break over them. They ascend and descend, so as to maintain their distance as the tides rise and fall; but will not bear long immersion in sea-water. (Couch.)

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**Family IV.—Limneidæ.**

(Pond-snails.)

Head with a short dilated muzzle; tentacles two, broad, short, compressed, not retractile; eyes sessile between their bases; mouth armed with an upper mandible; tongue-strap with teeth similar to Helix. Shell thin, horn-coloured; capable of containing the whole animal when retracted; aperture entire longitudinal; lip sharp, ascending towards the columella; apex sometimes eroded.

Genera—Limnea, Physa, Ancylus, Planorbis.

In the common pond-shell (Limnea stagnatilis) the apertures of the sexual organs lie far apart. Under the right feeler is the prepuce, evoking the penis; under the respiratory aperture is the vulva, or that of the female organs. To this it is to be ascribed that, in copulating, one individual is connected with two others, one of which impregnates it, whilst the other is impregnated by it. In this way, hanging together, they often form long chains. Von Baer has, however, observed self-impregnation also in Limnea auricularis. (Müller's Archie, ii., 1835, s.224.) The Limnean anatomy is well illustrated in Swammerdam, Bijbel der Nat. i., bl.164-169, tab. ix., fig. 4; Cuvier, Ann. du Mus. vii., pp.185-193, pl. x., figs. 2-11; Molussq. Mem. No.14; Stiebel, Dissert. inaug. sistens Limnei stagnatilis Anatom. Gottingen, 1815, 4to, c., tabulis 2.

The Limneids inhabit fresh waters in all parts of the world; they feed chiefly on decaying leaves, and deposit their spawn in the form of oblong transparent masses on aquatic plants and stones. They frequently glide beneath the surface of the water, shell downwards, and hibernate or estivate in the mud." (Woodward.)

Family V.—AURICULIDÆ, Woodward.

Mouth with a broad and short muzzle, tentacles two, cylindrical, the eyes sessile behind them; mantle-margin thickened; orifices as in the snails; foot oblong; sexes united; mouth with a horny upper jaw. Shell spiral, covered with horny epidermis; spire short, body-whirl large; aperture elongated denticulated; internal septum progressively absorbed.

Genera—Auricula, Conovulus, Carychium, Marinula, Cassidula, Polydonta, Pedipes.

Mr Adams remarks,—“The habits of this family are somewhat variable. Marinula affects salt water only, and Pedipes lurks in the cavities of rocks and under stones exposed to the sea. Cassidula is amphibious, having been observed crawling on a sandy bottom in clear water at a depth of nearly two fathoms, as well as in mangrove-swamps and on the sea-beach. Curricula and Melampus live in damp situations near the sea, and on the muddy banks of rivers. Polydonta inhabits moist situations in woods near the sea, but is wholly of terrestrial habits, living on decayed vegetable matter, and crawling about actively after showers of rain. Alexia and Carychium abound in salt-water marshes.” (Zool. Samarang, p. 55.)

Section 2.—OPERCULATA, Férussac.

“The operculated land-snails are exceedingly like periwinkles (Litorinae), and chiefly differ from them in the situations they inhabit and the medium respired. They have a long truncated muzzle, two slender contractile tentacles, and the eyes are sessile on the sides of the head. The mantle-margin is simple, and the pulmonary cavity is situated on the back of the neck, and quite open in front.” (Woodward.) The sexes are distinct.

Family I.—CYCLOSTOMIDÆ, Woodward.

Shell spiral, and spirally striated; aperture sub-circular, with a simple peristome; operculum spiral.

Genera—Cyclostoma, Lam.; Pupina, Vignard; Helicina, Lam.

Only one British species of Cyclostoma (C. elegans) is known, but upwards of 80 exotics have been characterized from the south of Europe, Africa, and Madagascar. Nearly half of these have the whorls spirally keeled, and have been subgenerically separated under the name of Tropidophora.

Family II.—ACICULIDÆ.

Shell elongated, cylindrical, with a subspiral operculum, in most pellicid.

Genera—Acicula, Geomelania.

The two dioecious pulmonate families make a transition, in regard to generative characters, to the next great section of the Gasteropoda; but the modifications of the parts of the complex combined organs of the Androgynous Gastropods are manifold. In regard to their ultimate terminations or outlets, a common genital orifice is found in Doris, Thetis, Bulimus, Clausilia, and Limax, as in the Helix above described. In Lymnaea the sexual orifices are wide apart; in Planorbis and Physa they are situated side by side,—the male orifice in front, on the left side of the neck, behind the tentacle. In Bulla, Bulla, and Aplysia, the male orifice is under the right tentacle; the female one much further back. In Doridium the male orifice is beneath the left tentacle; the female one on the same side, but near the opposite end of the body. In Oxicidium the female orifice is situated close to the anus, at the posterior end of the body; the male orifice is beneath the right tentacle. In all cases where the male and female organs are separate, a furrow may be traced from one to the other; the penis, when erected, projects from the male orifice.

GASTEROPODA DIGECIA.

Order I.—HETEROPODA, Lamark.

(Vulcubranchiata, Blainville.)

Locomotive organ compressed, resembling a fin, but single and ventral in position; branchiae, when distinct, pectinate and pinnate, packed in a small compass, with the heart in a dorsal sac or in a symmetrical shell. Sexes distinct. These Mollusks all live in the sea, and usually swim with the fin-shaped foot upwards and the back downwards. They progress rapidly by the vigorous movements of their compressed tails, or by a fan-shaped ventral fin; and adhere to sea-weed by a small sucker placed on the margin of the latter. Forskål, to whom we owe the first description of this family of animals, gave them the name of Pterobranchia. De Blainville associating with them certain Pteropods, called the group Nucleobranchia. The existence of distinct sexes was discovered by Lesueur, Laurillard, and Milne Edwards.

There are two families of Nucleobranchiate Mollusks—the Firoliidae, with large bodies and small or no shells, and the Atlantidae, which can retire into their shells and close them with an operculum. Both animal and shell are symmetrical, or nearly so; the nucleus of the shell is minute and dextrally spiral.

Family I.—ATLANTIDÆ, Woodward.

The shell large enough to protect the body.

Genera—Atlanta, Porcellia, Bellerophon, Cyrtolites.

Genus Atlanta, Lesueur.

Sp. Atlanta Peroni, Les. (figs. 66 and 67); Atlanta Keranodromi, Les. (fig. 68); Atlanta gibbosa, Soul. (fig. 69).—The soft parts of Atlanta are divisible into a “somatal” and a “pallial” region. In fig. 65 the former or chief fleshy part of the body is out of the shell; the pallial or visceral part is in the shell, which latter is an appendage to it. The “soma” is divided into the cephalic A, pedial B, and the lid-bearing tail or opercular lobes e, f; the two latter having that distinct degree of development which characterizes the Heteropoda. The head, or cephalic lobe, includes the mouth-mass a, the tentacles b, and the eyes l; the foot is divided into the “fin” B and the “disc” d; the tail includes the “leaf” c (“expansion foliacée”), and the “lid” or opercle f, with its surface of attach-

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1 Gr., signifying an “odd or imperfect foot.” 2 So called because the respiratory and digestive organs form a sort of nucleus on the posterior part of the back. 3 See also Leuckart, Zoologische Untersuchungen, Drittes Heft, Giessen, 1854, Der Bau der Heteropoden, pp. 1-68. The shell of the *Atlanta*, besides the beauty and symmetry of its shape, purity of colour, and delicacy of texture, is remarkable for combining two conditions of shell-tissue; retaining a large proportion of the mouth, or last-formed part, in a soft flexible quasi-cartilaginous state, the rest of the shell being vitreous. The whole is thin and translucent. It is suborbicular, compressed (fig. 68), and convolute on the same plane; umbilicate on each side, broadly keeled, and striated subtransversely. The whirls are usually three in number, and are visible in each lateral umbilicus; the last whirl, much larger than the rest, is that from which the keel is developed. This is a single plate, with a fine groove of uncalcified matter along its free border, which subsides towards the shell-aperture. The lid or operculum (fig. 67) is vitreous, transparent, very thin, and marked by transverse striae of growth, of a shape conformable to that of the shell-aperture.

The mantle includes the visceral mass, and lines the shell which it has developed. Round the base of the visceral mass it forms a broad fold corresponding with the shell-aperture, anteriorly into a large breathing-pouch. The thickened border of this pouch is provided with tufts of vibratile cilia, and in the species, *e.g.*, *Atlanta Keranodrenii*, in which the shell-aperture is canaliculate, the border is similarly produced (fig. 65, h), as in the siphonobranchiate Univalves.

For a knowledge of the anatomy of the *Atlanta* science is mainly indebted to MM. Eydoux and Souleyet, from whose beautiful illustrations of it in the *Zoologie of the Voyage of the Bonite* the scheme in fig. 65 is chiefly derived. The viscera occupy the spire and spiral half of the last whirl of the shell; the rest of that whirl contains the branchial sac and mantle, and into it the whole of the soft parts of the animal can be retracted, and be there shut in by the operculum. In this respect the *Atlanta* differs from other Heteropods; and the operculum is a correlative peculiarity to the protective size and office of the shell.

The subspiral retractor muscle (fig. 65, k) arises from the convexity of the innermost whirl, expands as it approaches the outlet, and then divides into fasciculi, which diverge to the head, to the foot-fin and foot-disc, and to the tail; the parts are retracted in the same order, the head being first pulled in. The mouth a is terminal, with a circular lip; the glottidium α is covered by a tooth-strap supporting a median row of imbricated tridentate plates and lateral rows, each row including a transverse plate with a hooked apex and two sickle-shaped denticles; the teeth increase in size from before backwards; the glottidium is partially protruded by short protractor muscles, and retracted by antagonistic fibres; the dental plates being alternately raised, divaricated, and depressed in the alternate movements of the rasp-like organ. The secretion of two long cylindrical salivary glands n is poured into the beginning of the oesophagus. The oesophagus dilates into a fusiform preventriculus m, from which the alimentary canal, bending round the inner whirl, penetrates the liver, and expands into a subquadrate stomach n, receiving the wide bile-ducts; the intestine o is reflected upon the dorsal aspect of the stomach, passes forward into the branchial sac, and terminates there on the right side, at some distance from the anterior border of the mantle. The short intestine is lined by ciliated epithelium. The chief mass of the blood, which, from the translucent delicacy of the tissues, seems to be diffused through the common cavity of the body, is contained in venous sinuses so disposed as to regulate the flow of the venous blood to the respiratory surface, to the filaments or lamina i of the branchial chamber h. Thence it is transferred and subjected to the action of the renal excretories q before it enters the auricle x, which is at the bottom of the branchial cavity. The ventricle s distributes the depurated blood by two principal arteries,—one to the visceral mass, the other reflected forwards beneath the alimentary canal, and supplying the foot and the head.

The nervous system includes two cephalic or superoesophageal, two pedal or suboesophageal, and a small visceral ganglion; the acoustics are connected with the cephalic ganglia or auditory sacs. The eye is lodged in its proper tentacle, whose clear convex end forms the cornea. The optic nerve, penetrating the inner wall, expands into a retinal ganglion, in front of which is a spherical lens; the ganglion and back part of the lens are covered by black pigment, and the whole is inclosed by a sclerotic which is continuous with the corneal part of the tentacular eye-sac. The acousticle has a small rotatory otolite. In the male the testis, usually more or less lobulated, occupies, with the liver p, the spire of the shell. It contains spermatozoa and spermatozoids. Its duct communicates with a dark-coloured accessory gland r, and a glandular dilated part or vesicle y, whence the sperm-duct is continued a short way forwards, and terminates in a ciliated groove leading to the base of the penis z, which is attached to the right side of the body. In the female the ovary occupies a corresponding position to that of the testes; the oviduct is continued from the fore part of the ovary; it is wide and convoluted, and terminates near the vent in the pallial cavity. Of this genus, presenting the organization above defined, many species have been characterized, chiefly by modifications of the shell. In some—*e.g.*, *Atlanta Keranodrenii*, Leueur (fig. 68)—the keel is continued to the aperture of the shell which is canaliculated anteriorly, with a corresponding siphonic modification of the mantle. In others—*e.g.*, *Atlanta Péronii* (fig. 66)—the keel subsides before attaining the aperture of the shell, which is deeply fissured.

In a few species—*e.g.*, *Atlanta gibbosa* (fig. 69)—the symmetrical form begins to deviate into the more common type of spiral Univalves; the shell of the embryo forming several obliquely spiral whirls before the particular generic type is resumed by the last large whirl of the adult. Thus the spire projects from the right side, leaving an umbilicus only on the left side; and the apex of the spire is turned forwards.

To this interesting type belong also the *Atlanta involuta*, E. and S.; *Atl. fusca*, E. and S.; and *Atl. turricula*, D'Orbigny. Nomenclature naturalists have not been wanting in the imposition of distinct subgeneric names upon the groups of the genus.

These small, delicate, and beautiful Mollusks are common in the high seas of tropical and temperate latitudes, floating or swimming by vigorous strokes of their fin above unfathomable depths. Such of their frail shells as may reach the bottom will form one of the few evidences of life that deposits from deep seas are likely to reveal; and certain delicate, finely-striated shells, analogous, if not allied, to *Atlanta*—such, *e.g.*, as *Eccidiomphalus Bucklandii*, Portlock, and *Bellerophon bicarinatus*, Leveille—have been found fossil in both Cambrian and Silurian rocks.—the most ancient

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1 The details given in the *Philosophical Transactions* for 1853, p. 36, are little more than confirmations of the facts made known by the able French naturalists whose anatomical labours are passed over, except in regard to "a small ganglion," which the writer states is "not figured by Eydoux and Souleyet," but which they nevertheless have not omitted to specify and figure in allied Heteropoda.

2 The auditory vesicles were first pointed out in *Heteropoda* by Souleyet; *Annales Français de l'Anat. et de Phys.*, tom. iii., 1841, p. 305; afterwards by Leydig; *Anatomisch. Bemerkung. üb. Carinaria, Perola und Amphibera*; *Zeitschr. fur wissenschaft. Zool.*, iii., 1851, p. 228.

3 See Zoologie de la Bonite, p. 362. of fossiliferous formations. Lamanon, the naturalist of La Peyronie's unfortunate voyage, believed that he had discovered, in the *Atlanta*, the living type of the *Corona Ammonis*; and the resemblance to the compressed and carinate species of Ammonite is undoubtedly very close; but it is one of analogy merely.

**Family II.—FIROLIDÆ**, Woodward.

A small branchial shell or none.

*Genus Carinaria*, Lam.—The next step in the departure from the common type of Univalves is made by the *Carinaria*.

Sp. *Carinaria cymbium*, L. (fig. 70).—In this species the somatal (fig. 70, A, B, C) far exceeds the visceral (D) division of the body, and the shell E is correspondingly reduced in size and adapted only to cover the visceral sac circumscribed by the free border of the mantle h h. Of the three divisions of the "soma," the cephalic A is the largest, the pedial B the least. The discoid portion d' of the foot is less distinctly separated from the pinniform part d than in *Atlanta*, and the tail C bears no operculum, the small shell being closely filled by the pallial and visceral mass, and affording no protection to the soma. The somatal integument is lined by a strong layer of decussating muscular bands, which layer is much more distinct and better developed than in other Heteropods; in the fin the muscular bands constitute four distinct layers: those of the two deeper-seated ones crossing the superficial layers obliquely. In the mid-space between the right and left layers lies the cellular tissue with the pedial vessels and nerves. The cephalic ganglions u are connected with the pedial ganglions v by commissural "cephalo-pedial" cords of a length corresponding with that of the cephalic division of the body, and consequently forming a so-called "oesophageal collar" of much greater extent than in *Atlanta*. Besides branches from this collar to the fin and tail, one is sent to the visceral ganglion x, and this brings the fin in relation with the branchial, circulating, and cephalic influences. The cephalic ganglions supply the same parts as in *Atlanta*, communicate with the small buccal ganglion, and send a nerve to the visceral ganglion x, situated at the base of the pallial division of the body.

The mouth is provided with a large rasping glottidium a' armed as in *Atlanta*; the oesophagus presents a fusiform dilatation m, in its course to the liver p; the intestine n describes a curve in that organ; the intestine is reflected dorsad and forward to terminate (o) at the fore-part of the pallial division, a little to the right of the median plain. The salivary glands f form a pair of caecal tubes, relatively shorter than in *Atlanta*. The liver p, of a violet colour, fills the hinder half of the shell. The branchiae i are more constant and are relatively larger than in *Atlanta*; in the form of pinnate processes projecting from the fore-part of the shell-aperture. The heart is composed of an auricle and ventricle, the latter overlying the former, and giving origin to a large aorta t. The testis w, of a gray colour and granular texture, lies in the upper and back part of the visceral mass, sends off a sperm-duct, which dilates at its commencement into a nodular receptacle y, lodged on the substance of the liver, and then extends, to terminate in the groove which leads along the right side of the animal, to the base of the penis z. This consists of two non-retractile appendages, hanging from a common base of attachment to the right side of the body; one of these is grooved lengthwise, and is the true intromittent part; the other has a granular structure within, and seems to be an excitory organ.

The *Carinaria Mediterranea* of Lamarck (a synonym probably of *Cor. cymbium*, Linn.) takes its name from the sea which it inhabits. The *Carinaria vitrea* is a native of the Indian ocean; its shell is highly prized.

In a closely allied genus, called *Cardiopoda* by D'Orbigny and *Carinarioides* by Eydoux and Souleyet, the shell is flexible, convolute, with the cartilaginous peristome expanded and bilobed in front, and enveloping the spire behind. The two figures above the part of cut 70, opposite the right hand, exemplify the shell, that on the opposite side is the shell of *Carinaria*.

*Genus Firola*, Péron and Les.—As amongst the land Gastropods the snails have a house-shell, the slugs a mere heart-shield, whilst in *Oncidium* and *Philomycus* even this rudiment of a shell disappears; so in the present sea Gastropods, so singularly modified for a free swimming, or floating life in mid-ocean, while one genus has a shell developed to include the whole body, and another has it reduced to a mere protective covering of the heart and a few other viscera; a third genus, with scarcely any other organic modification of importance, shows no trace of shell.

These parallel conditions of variability in the testaceous appendage of the skin in different natural groups of the Molluscous class show plainly the low grade of that appendage in the scale of organic characters, and the relative standing of pure Conchology in Zoological science.

The genera *Firola* and *Firoloides* exhibit the zero phase of shell development in the Heteropodous group. The pallial or visceral division of the body presents its least proportion, the cephalic as contradistinguished from the pedial part, its greatest proportion. The foot is now a mere appendage to the somatal division, from which the cephalic part, properly so called, can no longer be distinguished as in *Atlanta*. The general form of the body, the transparency of the integument, and its muscular layer, are very similar. to those in Carinaria; but some species of Firoloids have no tentacles (fig. 71), and in none is the disc developed on the foot; this is exclusively a fin in form and function. The mouth (fig. 71, a), and mouth-mass a b, extend far in advance of the cephalic ganglions u, and eyes c. The commissural cords connecting those ganglions with the pedial ones e, cross the sides of the inhalial dilatation of the gullet m, and form a very long oesophageal collar. The alimentary canal extends to near the caudal end of the body before it begins to make its molluscous inflection, and to expand into the gastro-billary sac, with the substance of the liver p. The very short intestine o ascends, forming merely a right angle with the axis of the trunk before it terminates in a vent o', just behind the renal sac and heart. From the ventricle a large aorta proceeds, and divides into a visceral and a somatal part; the latter advances forwards, sends off a small branch to the tail, a larger one to the foot, and is itself continued to the head. The pedial artery terminates abruptly in the reticulate venous sinus of the foot, and is said to exhibit a strong contractile power at its termination; whence it is probable that, when the foot is in vigorous action as a swimming organ, the flow of blood into it is free; but when the muscles are at rest, the current may be decreased or checked. From the general transparency of the tissues of the Firoloids the delicate membrana propria of the wide and effused sinuses by which the venous blood is guided, and finally returned to the auricle, has been overlooked. That such an essential part of the mechanism of circulation should therefore be absent, and the blood-corpuscles be doomed to wander back to the heart "as best they may," is an inference characteristic of the school which repudiates final causes (or purposive adaptations) in organic structures.

From the delicacy of the somatal tissues it may well be supposed that the oxygenated sea-water re-acts upon the generally diffused venous blood; and it is certain that in Firoloides proper, as in some species of Atlanta, branchial filaments are not developed, and the ciliated subspiral band represents the sole localized or special breathing organ. The renal sac is contractile, and alternately receives and expels sea-water. Two small salivary caecal tubes communicate with the mouth.

Lesueur first discovered the distinction of the sexes in the Firoloids. The testis (fig. 71, u), is a whitish granular body, situated at the upper and back part of the visceral mass. Its duct dilates into a receptacle (w) on quitting the gland, then contracts to a tube of capillary tenacity opening into the groove that is continued into the intromitent and excitatory appendage. This depends from the right hinder and lower angle of the body, and divides into two parts; the glandular portion terminating in a globular body. The ovary in Firoloides, as in Firoloides, terminates the small visceral mass behind; it usually presents ova, with a well-marked germ-cell and nucleus. The albuminous sheath of the excluded eggs is frequently seen hanging from the outlet of the short oviduct like a filamentary appendage.

The Firoloids are amongst the most abundant of the pelagic Mollusks; they float in myriads on the surface of the Mediterranean, and in the warmer zones of the Atlantic and Pacific Oceans.

In Firoloides proper, the visceral mass, answering to the pallial division of the body in Atlanta, is situated, as in Carinaria, at about one-fourth from the hinder end of the somatal division. This end terminates in a filamentary appendage, varying in size and shape in different species. The branchial filaments, from twelve to fifteen in number, project from the slight border or fold of the small mantle, which leaves the upper and back part of the visceral mass exposed. The transparent integument of the body has a few coloured tubercles scattered over it. A rudiment of a disc has been seen on the compressed foot of some Firoloides, but is not constant. A pair of small tentacles are present in some species, not in others; they appear, however, not to be constant in all the individuals of the same species. In fact, the determination of a species is by no means an easy or satisfactory operation in these soft, contractile, pelagic Mollusks.

The Firoloides Keradrenii has a finely granular skin, and a long filamentary caudal appendage; it is about 2 inches in length.

In Firoloides the visceral mass is terminal; only the short filamentary appendage projects beyond it; in Fir. Desmarestia, and in Fir. Lesueuria, the appendage is multarticulate, or antenniform. These delicate little Heteropods rarely exceed an inch in length; they occur in both the Pacific and Atlantic Oceans.

ORDER II.—TUBULIBRANCHIATA, Cuvier.

Branchiae two, symmetrical, behind the heart; inclosed with the other soft parts of the animal in a long shelly tube.

Gen. Dentalium, Linn.

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1 Prof. Huxley, Phil. Trans., 1852. 2 Journal of the Academy of Natural Sciences of Philadelphia, vol. i., p. 3; Firoloides is described, Huxley, Phil. Trans., 1852. 3 The idea of the posterior termination of the body being homologous with part of the foot of gastropods has been brought forward as a novel one by the author of the paper published in the Philosophical Transactions for 1853. But I concur in the justice of the following remarks by the distinguished naturalists of the Voyage de la Bonite—"C'est la fausse analogie que l'on a ainsi établie entre de simples expansions de la peau et les véritables organes locomoteurs, qui à surtout fait méconnaître les secrets naturels de ces Molusques." Zoologie de la Bonite, Svo, 1841-1852, p. 346. The part e (fig. 71) may be homologous with the hind part of the body from which the hind lobe of the foot is developed in Limae and Doris, but not with that lobe itself. 4 Ibid., p. 32. 5 Ibid., plate xvi., fig. 1. Branchiae usually a series of lamellae, surrounding the body between the foot and mantle.

**Family I.—Patellidæ**, Woodward.

Shell with the apex turned forwards.

*Genus Patella*, Linn. (*Limpets*).—Head with two acuminate tentacles, ocelli sessile, on the outer side of their base. Pallial margin fringed; branchial series continuous. Shell covering the body, oval, with a subcentral apex.

The tongue of the common British limpet (*P. vulgaris*) is rather longer than its shell; it has 160 rows of teeth, with 12 teeth in each row, or 1920 in all (Forbes).

The intestinal canal is very long and forms many convolutions, the rectum being reflected, so that the vent is placed under the head on the right side, whilst in *Chiton* it lies in the mid line at the posterior extremity of the body.

"The limpets live on rocky coasts, between tide-marks, and are consequently left dry twice every day; they adhere very firmly, by atmospheric pressure (15 lb. per square inch), and the difficulty of detaching them is increased by the form of the shell. On soft calcareous rocks, like the chalk off the coast of Thanet, they live in pits half an inch deep, probably formed by the carbonic acid disengaged in respiration; on hard limestones only the aged specimens are found to have worn the rock beneath, and the margin of their shell is often accommodated to the inequalities of the surrounding surface. These circumstances imply that the limpets return to the same spot to roost. On the coast of Northumberland we have seen them sheltering themselves in the crevices of rocks, whose broad surfaces, overgrown with mollusca, were covered with irregular tracks, apparently rasped by the limpets in their nocturnal excursions." (Woodward.)

The limpet is much used by fishermen for bait; on the coast of Berwickshire nearly 12,000,000 have been collected yearly, until their numbers are so decreased that collecting them has become tedious (Dr. Johnston). In the north of Ireland they are used for human food, especially in seasons of scarcity; many tons weight are collected annually near the town of Larne alone (Pattison).

On the western coast of South America there is a limpet which attains the diameter of a foot, and is used by the natives as a basin (Cuming).

(For the anatomy of the limpet see Cuvier, *Mémoire sur les Mollusques*, No. 18, pp. 15–19, pl. ii., figs. 8–19, and the fig. of *Patella algira*, Desh. in Cuv. *R. Ani.*, ed. ill., *Moll.*, pl. 66.)

**Family II.—Chitonidæ.**

Shell subdivided into eight pieces.

These pieces or valves form an imbricate series on the middle of the back (fig. 72, a). Each has a more or less wide plate of insertion sunk into the mantle, with a deep lateral notch on each side; the anterior and posterior valves seven plates have posterior apices (fig. 73, i–7); the eighth has its apex nearly in front. The six middle plates are each divided into a dorsal and two lateral areas. Gills conic, lamellar, in two series, one on each side the hinder part of the body. The heart is elongated like a dorsal vessel. Mantle not covered by shell at the circumference, with the margins hard, coriaceous, often aculeate or squamoso. Ventral disc elongate, narrower than the body (fig. 72, b). Eyes and tentacles none; head crested by a wavy veil. The combined genital organs are symmetrically repeated on each side, and have two orifices.

More than 200 species of Chitons are known; they occur in all climates throughout the world; are most abundant on rocks at low water, but frequently obtained by dredging in 10–25 fathoms water. Some of the small British species range as deep as 100 fathoms (Forbes).

The species of this genus are very numerous and difficult to distinguish otherwise than by accurate measurements of the comparative length and breadth, the greater or lesser inequality, &c., of the pieces of shell, and by the nature of the margins of the mantle which are not covered by the dorsal shields. In some species these margins of the mantle cover, as though they came together by continued growth, the dorsal shields, so that the shell is concealed within them. Of these Middendorff forms the subgenus *Cryptochiton* (Sp. *Cryptochiton Stelleri*, Middend., l. Taf. i., figs. 1, 2, *Chiton amicus*, Sowerby, *Conchol. Illustrations*, *Chitones*, fig. 80, Reeve, *Conchologia system.*, ii., Pl. 132, 133, fig. 80; from which *Chiton amicus*, Pall., according to Middend., does not differ). The remaining species form the sub-genus *Phenochiton*, Middend., in which the branchiae are sometimes situated backwards, and the row of pieces of shell is interrupted by the mantle (*Dichachiton*, Middend., *Chitonellus*, Lam., Sp. *Chitonellus lacis*, Lam., Blainv. *Malac.* pl. 87, fig. 5); or the pieces of shell close upon one another, and the branchiae are placed round about at the sides of the mantle (*Hamachiton*, Middend.). To this last division belong most of the species of the genus *Chiton* of Lamarck.

Dr Gray has called attention to the value and constancy of the modifications observable in the inserted part of the plates, and this experienced naturalist remarks:—

"It has been objected that the character derived from the form of the plate of insertion of the valves can only be seen by the destruction of the specimens as they are usually kept in cabinets; but they can generally be seen from the under side, or through the substance of the mantle. When this is not the case, the form of the plate of insertion can be easily developed by paring away the under surface of the mantle, so as to show part of the edge of the valves, without injury to the specimen; and they may be easily made more visible through the inner side of the mantle by being soaked for a few hours in a weak solution of caustic potash; but care should be taken that the specimen is not left too long in soak, nor the solution be too strong, otherwise the margin will be dissolved.

"The form of the plate of insertion may also be easily predicted by inspecting the inner surface of the valves, for the notches in the margin leave an impressed line from the vertex of the valve, as they are gradually filled up by the growth of the valve.

"The valves are best separated from the coriaceous skin of the body, called the mantle, by soaking them in a strong solution of caustic potash; as then the plates of insertion are cleaned, and not broken, which they are likely to be if they are taken by force from the mantle.

"The number of notches in the plates of insertion is sometimes, but very rarely, liable to variation. In one specimen of *Chiton Bovceni* in the Museum Collection, the plate of

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1 Mémoires de l'Acad. Imp. des Sciences, Petersbourg, tom. vi., 1848. insertion of the last valve but one has two notches on one side, but the normal single notch of the genus on the other."

The anatomy of the Chiton has been illustrated in the great work of Poli, Testacea utr. Siciliae i. Multicola, pp. 1-10, tab. iii.; by Cuvier, Mem. pour servir à l'Hist. nat. et à l'Anat. des Mollusques, No. 18, pp. 22-23, pl. iii., figs. 8-14; and, more recently, in an exhaustive manner, by A. Th. Middendorf, Beschreibung u. Anatomie neuer Chitonen, Mem. de l'Acad. impér. des Sc. de St Pétersbourg, 6e série, tom. vi., 1848, pp. 67-215, av. 14 pl.

The chief peculiarities of organization have been strikingly summed up, with perhaps some overstretching of the similes, by Dr T. Williams, as follows:

"A Chiton has a carapace like an isopod Crustacean, a dorsal vessel like an Annelid, bilateral symmetrical reproductive viscera like an acephalous Mollusk, a head and foot like a patellid Gastropod, a posterior anus like the Fissurella, and branchiae like those of the brachyurous Crustacea! Such manifold affinities at once unite and sever this odd group from several most dissimilar classes." (Dr T. Williams, Ann. & Mag. Nat. Hist., xvi., 408, 1855.)

The fry of the Chiton are not provided with a deciduous shell: they swim by a cincture of long vibratile cilia, near a constriction which divides the oval embryo into two subequal parts; a tuft of filaments and two dark eye-specks denote the head. The hinder half elongates, becomes marked on the back with seven transverse furrows, between which the first rudiments of the shell-plates appear as granules of carbonate of lime in the integument, which coalesce into narrow bands or bars, with an irregular wavy outline: they increase by the addition of layers on the under surface: and their development gives no countenance to the idea that the hindmost valve is the homologue of the univalve shell of the limpet, and the seven in-front supplementary pieces. Lovén, who has described the development of the Chiton, regards the girdle of cilia as answering to those on the velum of other Gastropods; the homology of the head of the embryo, with its tuft of filaments, to the prominence supporting the flagellum in some embryo bivalves (fig. 27), is more doubtful.

**Order IV.—SCUTIBRANCHIATA**, Cuvier.

Branchiae plumose or pectinate, and, with the entire body, protected by a widely-open shield-shaped inoperculate shell. Heart with two auricles, and traversed by the rectum.

**Family I.—Haliotidae.**

*(Ear-shells.)*

Head with two long and slender tentacles; eyes on short pedicles at the outer side of their base; proboscis short; branchial cavity with a fold or siphon occupying the slit or perforation of the shell; retractor muscle large, subcentral.

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**Genus Haliotis**, Lam. *(Ormers or Ear-shells).*—Shell ear-shaped, with a spiral ridge on the left side, and a series of holes over the pallial fissure. These holes are closed as the animal grows from near the apex of the shell onwards, new ones being formed near the aperture, where the notch becomes changed into a hole. The foot (fig. 74) dd is expanded and sub-circular, its side lobes support a double row of cirri and crenate processes; cirri project through the perforations of the shell i, i.

*Haliotis tuberculata* (fig. 74) abounds in the Channel Islands, where it is used for food; the hard tissue of the muscular foot, dd, and retractor being pounded before the animal is cooked.

Species of *Haliotis* range from Kamtschatka to New Zealand. In the latter island is a sub-genus (*Padolus*), in which the multi-perforate shell has two parallel spiral ridges.

The lining nacre of the ear-shell is beautifully iridescent; most richly so in *Haliotis iris* and *Haliotis gigas*, which latter shells are imported in vast numbers for that substance.

**Genus Janthina**, Lam. (fig. 75).—Head with a large ventricose proboscis a; two subulate tentacles b, with short eye-stalks d at their base; foot f small, with lateral folds g continuous behind. It secretes (in the female?) a series of albuminous air-vesicles i, constituting a floating raft, to the under surface of which the ovi-capsules mm are attached. M. Rang states that this float is cast adrift after the egg-bags are attached. The shell n of the Janthine is like that of a land-snail, and commonly of a beautiful purple colour; it is ventricose, thin, and sub-pellucid, with the aperture triangular, and the columella straight and produced beyond the margin of aperture. Myriads of these beautiful and delicate oceanic snails float in the Mediterranean, Atlantic, and Indian Oceans; specimens are occasionally drifted to the S.W. coasts of England. They are said to feed on the small floating Veletta. The anatomy of *Janthina* has been given by Cuvier in the Ann. du Mus. xi., pp. 121-130; and in the Mollusques, Mem. 15, pp. 2-10, figs. 1-8.

**Family II.—Fissurellidae**, Woodward.

Head with a short proboscis; with two subulate tentacles, having sub-pedunculate eyes on the outside of their base. Mantle with a siphonal aperture in the back, and containing two symmetrical pectinate gills. Foot expanded, with a series of short cirri above its sides. Shell symmetrical, but with a spiral nucleus, which is covered in most by later growths of the shell; the apex perforated corresponding to the branchial opening.

**Genus Fissurella**, Lam. *(Key-hole Limpets).*—Shell oval, conical, radiately ribbed; apex subanterior or central; perforation oblong, subsapical; aperture wide, oblong; cavity simple, muscular, impression with the points incurved. The shell of the young animal is subspiral, with a perforation in front of the apex. If the branchial excretory fissure were extended so as to bisect the shell, and the two symmetrical halves were connected by a hinge and adductor muscles, the Fissurella, with its symmetrical pair of gills, would closely resemble a Lamellibranch, with a head and a very large and broad foot.

In *Emarginula*, Lam., the vent is anterior, and the shell presents a slit at its front margin; the foot supports a single cirrus on its back part. In some species of *Emarginula* the shell can be partially covered by the mantle. The young of *Emerginula fissura* have a very minute, recurved, spiral shell, with rapidly enlarging and finely striated whirls without any slit. (Lovén.)

**Order V.—PECTINIBRANCHIATA**, Cuvier.

Branchiae composed of plates so formed and united as to resemble a comb, usually two in number, very rarely obsolete; contained in a dorsal pallial cavity opening widely above the head. This order of bivalve Gastropods is the most numerous in genera and species; it includes nearly the whole of the spiral Univalves, and also many with shells simply conical. All the Pectinibranchiata have two tentacles and two eyes, in many supported upon pedicles; the mouth is produced into a proboscis; the males have the intromittent organ attached to the right side of the neck, and, in most, it is folded back within the pallial cavity, but in *Paludina* it is retracted into a prepuce opening at the right tentacle. The female has a nidamental gland excreting the albuminous matter of the nidus for the eggs, which is often large and complex. In some Pectinibranchs the branchial aperture is entire; in others a portion of it is fissured or produced into a siphon; the mouth of the shell presents conformable modifications; in some the shell is without an operculum, but most Pectinibranchs possess one. In the selection, compelled by the limits of the present article, the chief modifications of the soft parts or "animal" of the univalve shells will be illustrated.

**Section I.—ASIPHONATA.**

**Family I.—CALYPREIDÆ**, Woodward.

*(Bonnet and Slipper Limpets.)*

Head with two tentacles, having small sessile eyes on the outside of their base; branchia single and long; sides of the body in front of the foot more or less expanded and produced; tooth-strap elongate; teeth 3–1–3; the central simple, recurved, toothed at the tip; the inner lateral broad, with a triangular toothed tip; the outer lateral tooth claw-like; the second lateral minutely toothed at the end. The eggs are inclosed in a thin membranous bag in small groups under or in front of the foot of the animal. The shell in the egg is subglobular, of one or two gradually enlarging whirls, which rapidly enlarge as soon as the animal is hatched. Shell limpet-shaped, but with the apex more or less spiral, and with a variously-shaped calcareous process from the inner surface to which the retractor muscle is attached.

**Genus CALYPREA**, Lam.—Head with a short and broad muzzle (fig. 77, a), truncate, and with the angles slightly produced; tentacles b rather short; aliform expansions d from the sides of the neck (in *Calypeopsis*). Shell with the inner process half-cup-shaped (*Calyprea*), or cup-shaped (*Calypeopsis*) (fig. 76, b), or with a second calcareous plate formed by the surface of the foot, and adherent to the foreign base of attachment (*Lithedaphus*). The anatomy of a male *Calyptraea Byronensis* is shown in fig. 77. a is the mouth, k the glottidium with the tongue-strap and teeth as above described; ll the simple elongated salivary follicles; m the oesophagus girt by the nervous collar before its expansion into the stomach n; this cavity is almost concealed by the liver, in the substance of which the intestine makes an abrupt bend upon itself and returns, crossing the dorsal side of the stomach to terminate in the right side at p; close to the branchial or pallial aperture h. The pallial cavity is laid open and spread out to show the extent of the single comb-shaped gill ii. The great branchial vein x enters the auricle w, and the small, dark, muscular ventricle propels the blood through a short bifurcating sorta. The testis r sends its product by the duct d to the base of a long and slender grooved intromittent organ c, projecting from the right side of the head. f shows the smooth convex mass, chiefly muscular, which fills the "cup" (fig. 76, b); the dark colour indicates the shape and position of the insertion of the adductor muscle. These singular "cup-and-saucer" limpets adhere to rocks or shells, to the inequalities of which the margin of the "saucer" adapts itself, indicative of their sedentary habits. *Lithedaphus* is a fixture.

In one group the shell is elongated, and the inner process is a partition covering the hinder half of the cavity like a slipper, whence the name *Crepidula* (little slipper) given to this group or genus.

The shells of all the *Calypreidae* are peculiar for their great range of adaptive modifications, which have given rise to many nominal species. The experienced naturalist Dr Gray remarks:

"Crepidula formicata varies in shape according to the body on which it rests; four or five different ages are frequently found riding upon each other. When growing on *Pecten concentricus* it is found to have ribs corresponding to those of the *Pecten*. When found in the cavities of shells it is white and flat, with a convex diaphragm; the younger specimens are rounded, but generally become elongated by age; and it is otherwise very variable in shape, conforming itself to the position it occupies in the throat of the shell. When found among stones at the roots of seaweed, it is generally of a small size, with a convex diaphragm.

*Crepidula dilatata* changes its form and character according to the body to which it happens to be fixed:

1. If attached to a broad stone or other body, the shell is smooth, circular or ovate, and moderately convex; it is then *C. dilatata*, *C. perspicua*, *C. depressa*, *C. patula*, *C. lineolata*, *C. Adolphae*, *C. chilensis*, and *C. strigata*.

2. On the contrary, if fixed to a small round stone, and... the shells are obliged to group themselves on one another, it is then narrow, very convex, and *C. nautiloides*.

"3. When the young shell happens to be in deeper water, and fixes itself between the roots of sea plants, it becomes irregular, thick, and of a uniform colour; it is then *C. pallida*.

"4. If it happens to be fixed on the inside of a dead spiral shell, especially if that shell is inhabited by a soldier crab, it becomes flat or even concave externally, and is of a white colour; it is then *C. plana* or *C. unguiformis*.

*Crepidula patagonica* and *C. aculeata* occur attached to stones on the coast of South America; the former often chooses situations much beaten by the waves.

*Crepidula dilatata* forms large rounded massive groups, the lower shell being attached to a stone or shell, and the others all placed on the back of it."

**Family II.—TURBINIDÆ**, Woodward.

In this family, as in the *Bullidae*, the head and sides of the foot bear lobes, but they are fringed and tentaculate, as in *Haliotis*; the eyes are pedunculated outside the base of the tentacles. The gill-comb is single; the tongue-strap is very long, extending into the visceral cavity. Like the limpets, the Gastropods of the present family feed on seaweeds; they include the fabricators of the top-shells (*Turbo* and *Trochus*) and pheasant-shells (*Phasianella*) of the Conchologists; nearly all of which have a brilliant nacreous lining. The operculum is either shelly, or is horny and multispiral. The *Trochus conchyliophorus*, Born., Gm. (Cuv. *Régne Animal*, éd. ill., *Moll.*, pl. 41, fig. 3), causes little stones and pieces of bivalves from the bottom on which it creeps to adhere to its shell; these foreign bodies in time grow to the shell, which thus acquires a very irregular appearance. Lamarck confounded with this species, from the West Indian Seas, a fossil species from the tertiary formations, which presents the same peculiarity, under the name of *Trochus agglutinans*. The anatomy of the *Trochus* is given by Cuvier, in *Ann. du Mus.* xi., pp. 184, 185; *Mollusques Mém.* 16, pp. 15, 16, fig. 13.

**Family III.—PALUDINIDÆ**, Woodward.

The Gastropods of this family inhabit fresh waters, and have a world-wide distribution. Our well known river-snail (*Paludina vivipara*, Lam.) may be regarded as the type; the females are full of young in spring. The ova are hatched in a long and wide oviduct, and the young escape with shells, the periostracum of which bears spiral rows of cirri. The shell of the adult is smooth, of a dull-green colour, with red-brown bands and very convex wreaths. The right tentacle is perforated in the male individuals with an aperture which gives passage to the penis. The gills consist of three series of filaments. It is found in rivers, canals, and other fresh waters; in the mud, or hiding under stones; also creeping under water-plants. The anatomy of the *Paludina vivipara*, Lam., is given by Swammerdam, *Bibl. Nat.*, i., pp. 169–180, tab. ix., figs. 15–18; by Cuvier, *Ann. du Mus.* xi., p. 170, *Mollusques Mémoire* 15; and it has recently been admirably completed by Leydig, *Ueber Paludina vivipara, ein Beitrag zur nöthigen Kenntniss dieses Thieres in embryologischer, anatomischer und histologischer Beziehung*, in Siebold u. Kölliker's *Zeitsch. f. wissensch. Zoologie*, bd. ii., 1850, pp. 125–197, pls. xi., xii., xiii.

**Family IV.—LITORINIDÆ**, Woodward.

Head proboscidiform (fig. 78, a); tentacles b subulate, with the eyes sessile on the outside of their base; foot of moderate size, with a linear duplication in front, and a groove along the repent surface; gill single; mantle with a rudimentary siphonal canal. Shell spiral, turbinated, or depressed; aperture entire; operculum horny.

*Genus Litorina*, Féussac (*Periwinkles*).—Shell turbinated, thick, with few whorls, and no nacreous lining; operculum paucispiral.

Sp. *Litorina litorea* (Common Periwinkle).—This well-known univalve lives in the lowest zones of sea-weed between the tide-marks; it is oviparous, and is used for food.

*Litorina rudis*.—This species is viviparous, and as the young acquire a calcareous shell before exclusion, it is not eaten; it frequents a higher zone than the common periwinkle. The sides of the grooved foot advance alternately in progression.

*Genus Solarium*, Lam. (*Stair-case Shell*).—To a subgenus of this group (*Torinia*, Gray) belongs the Mollusk with the spiral turiform operculum, fig. 56.

*Genus Phorus*, Montfort (*Carrier-Shell*).—The propriety of separating the "carrier-shells" from the *Trochus*, which they resemble in shape, has been proved by the discovery of two living species, in both of which the soft parts are distinct from those of *Trochus* proper. Except in that the eyes are not raised on pedicles, the outward form of the animal is similar to that of *Strombus*, which *Phorus* resembles also in its mode of progression, but its true affinities are with the *Litorinidae*.

The animal of *Phorus* (fig. 78) is very slender in proportion to the size of the aperture of the shell. The foot is divided into two parts, of which the anterior one d is expanded, subservient to the purposes of locomotion, and the hinder one supports a large horny operculum f, which is partially free, as in *Solarium*. The proboscis a is very prominently developed and annulate; and the tentacles b are long and tapering, with the eyes c sessile on the outside of their base. The portion of the mantle lining the aperture of the shell is vascular, thin, and delicate, extending over the front and outer lip, which is often much produced and uneven in outline, especially in *Phorus excutus* (fig. 78).

"The *Phori* invariably inhabit rough places incapable of accommodating a gliding motion, and their mode of progression, like that of the *Strombi*, is by little jumps. Each species has its own peculiar manner of collecting the debris of shells or pebbles which cover the ground it inhabits, and each has, to a certain extent, its peculiar kind of debris." (Adams.)

In *Phorus Solarioides*, Adams, the animal is character-

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1 Guide to the Systematic Distribution of the Mollusca in the British Museum, 8vo, 1867, p. 117. ized by numerous circular striae, the tentacles are laterally compressed and rather prismatic, the proboscis is long and transversely wrinkled, yellow at the tip and on the under surface, but pink between the tentacles, which are straight, rigid, and opaque dead-white; the eyes are black and conspicuous. Hab. China Sea.

As compared with the *Calyptraeidae* the carrier-shell have a divided Stromb-like operculated foot, are of active habits, and produce a regular convoluted shell, whilst the cup-and-saucer shells have a simple foot, live attached to foreign bodies, seem rarely to perform the act of progression, have no operculum, unless the rare secretion of a calcareous plate to the place of attachment can be regarded as the homologue of one, and the spiral type is not carried out in the completion of the adult shell.

**Family V.—Turritellidae**, Woodward.

Head with a short proboscis; eyes sunk in the outside of the base of the tentacles; margin of the mantle fringed; foot short; branchia single. Shell tubular or spiral, apex partitioned off; aperture circular, entire; operculum horny multispiral. To this family belong the "turret-shells" (*Turritella*, Lam.), the "worm-shells" (*Vermetus* and *Magilus*), and the wentle-traps (*Scaloria*, Lam.), of which the once rare and highly-prized *Scaloria pretiosa* is a beautiful example. The wentle-traps exude a purple fluid when molested.

In the *Vermetus* of Adanson (the *Serpula lumbicralis* of Linnaeus) the tubular shell is thin, divided by partitions, with the spire adhering at the apex; elsewhere it is loose. The aperture is orbicular, with the margins connected. The head supports two tentacles, which are oculiferous externally at the base. The posterior appendage of the body is bent downwards, and is mostly furnished with an operculum. The branchiae are arranged in a conical series, at the left side of the respiratory cavity.

The peculiar solidification of the elongated shell of the adult *Magilus* has already been mentioned. In both the above genera of tube or worm shells the shell has the form of a regular spiral univalve when the animal is young. *Magilus* inhabits madreporic masses in the Red Sea; by its anatomy it is nearly allied to the whelk family (*Buccinidae*).

**Family VI.—Cerithiidae**, Woodward.

Head with a short non-retractile muzzle; tentacles with the eyes at their apex (fig. 79), or supporting them on short peduncles; margin of the mantle with a rudimentary siphonal fold. Shell spiral, elongate, and many-whirled; aperture oval, oblique, with a short truncate or recurved canal in front; operculum horny and spiral. About 100 living species of *Cerithium* are known; they have a worldwide distribution, but flourish best in the tropics. They live in great part in the sea, partly in brackish water or at the mouths of rivers. The fossil species, which are still most numerous (upwards of 400 have been defined), are almost all limited to the Tertiary formations. Amongst these *Cerithium giganteum* deserves to be noticed, a species found in France near Grignon.

The animal of *Cerithium obtusum*, Lam. (fig. 79), captured by Mr Adams at the mouths of rivers in Borneo, "has a broad, suborbicular, and expanded foot, and an elongated, subcylindrical annulated trunk of a light brown colour, with three rather broad, well-defined, opaque, yellow lines extending along its upper surface, the central one of which extends from the head to very near the extremity of the proboscis, where it is bifid, the two forks diverging; the two lateral lines are shorter, not bifid at their extremities, and reach forward on the head to within a little distance of the origin of the tentacles; the tentacles are very short, annulated, with the eyes (which are small, though with a distinct iris and pupil) situated at their tip, whereas they are mostly placed on tubercles situated on the outer side of the base of the tentacles, or on the tentacles themselves at a little distance from their origin. The foot is of a light pinky brown on its upper surface, mottled with a deep rich brown, and on the under surface is lilac."

The *Cerithium obtusa* live in brackish water in mangrove-swamps and the mouths of rivers in Singapore and Borneo. Sometimes they crawl on the stones and leaves in the neighbourhood, and are not unfrequently found suspended by glutinous threads to boughs and the roots of the mangroves. The operculum is round, horny, with a central nucleus and concentric elements; it is semitransparent, and borne upon the posterior part of the foot at its extreme end. When the animal hibernates it retracts itself into the shell, and brings its operculum to fit closely into the aperture, after having previously affixed sixty or seventy glassy, transparent, glutinous threads to the place of attachment, when they occupy the outer or right lip, and extend half-way round the operculum.

"A species of *Cyclostoma* (*Megalomastoma suspensum*, Goulding) was found by the Rev. Landsdowne Goulding at the Island of St Vincent, suspended in like manner from the trees; and *Rissoa parea* has been observed by Mr Gray, upon our own shores (*Proc. Zool. Soc.*, 1833, p. 116), to have the power of emitting a glutinous thread by which it attaches itself to floating sea-weeds.

"There is a very handsome *Cerithium* closely allied to the foregoing, which I have frequently found crawling languidly on the leaves of the *Pontederia* and sedges in the fluvial marshes on the banks of the rivers in many parts of Borneo, and many miles in the interior where the water is perfectly fresh, and which has the eyes likewise terminal and the proboscis marked with crimson and yellow; the foot is very dark brown, and has a vivid scarlet line extending round the lower margin. The position of the eye varies considerably in this group. In an amphibious Borean species, allied to *C. decollatum*, they are terminal at the end of peduncles; in other words, the tentacles are connate with the eye-peduncles for the whole of their extent. In *C. microptera* the tentacles extend a third beyond the eye-peduncles; in *C. decollatum* the eye-peduncles are truncated, with the eyes at the end, while the tentacle extends beyond them in the form of a minute filament; all these species have circular multispiral opercula."

The foregoing evidence of the living habits of the *Cerithium* has much interest in relation to the tertiary extinct species.

**Family VII.—Pyramidellidae**, Woodward.

Head with broad ear-shaped tentacles, often connate, with the eyes at their base; proboscis retractile; foot truncate anteriorly. Shell spiral, turreted; aperture small; columella with one or more prominent plaits; operculum horny, imbricated.

*Genus Eulima*, Risso.—"The head of *Eulima* is small, with the tentacles subulate and close together at the base, where they are rather swollen. The eyes are situated at the back of the head, behind the tentacles. The foot is rather expanded, especially at the sides, and is furnished with an ovate subspiral operculum. The polish of the shell is occasioned by the front edge of the mantle being..." extended over it; the lobes are, however, difficult to observe, in consequence of the extreme timidity of the animal in speedily retracting them when disturbed. The soft parts of Bulimia major are, like the shell, of an opaque pearly white, except that the tentacles are delicately tinged with orange in the middle and with yellow at the tip. The eyes, which are black, are usually concealed beneath the front of the shell, the tentacles only protruding. The foot is in advance of the head when the animal is creeping.

Genus Styliina, Fleming (Stylifer, Brodt.)

Sp. Styliina astericola.—This little Gastropod is remarkable for its parasitic habits, being commonly found immersed in the body of living star-fishes, or attached to the spines of sea-urchins. The mantle is thick and reflected over the body-whirl of the shell. The foot is much produced beyond the head, but is very little extended behind.

Family VIII.—Naticidae, Woodward.

Animal with a long retractile proboscis; tentacles often connate with an expansion of the head; foot large; mantle-lobes reflected over more or less of the shell; this is globular and with few whorls. Spire short and obtuse; aperture semilunar.

Genus Natica, Lam.—The Natica melanostoma (fig. 80) is furnished with a strong coriaceous foot d, well developed in front, where it forms a fold a, which can be reflected back upon the head. By means of this the animal perforates the sand, while its tentacles are protected; but when the tide rises and covers the sands, the large side lobes and dilated hind part of the foot are expanded, and the Natica flaps along above the sand. A great peculiarity in the animal of this family is the existence of an opercular lobe h, which in the polished species nearly covers the shell, and is seen in our figure mounting up behind and partly covering the sides. In Sigaretus this lobe is extended entirely across, and covers the shell, while the operculum is rudimentary; in Lamellaria it not only incloses the shell, but extends beyond it in front.

Section 2.—Siphonostomata, Cuvier.

Margin of the mantle prolonged into a siphon, by which water is conveyed into the branchial chamber. Shell spiral; aperture notched or produced into a canal in front; operculum horny, lamellar, rarely wanting. Most of this section are carnivorous.

Family I.—Cypraeidae.

(Cowries.)

Head with a proboscis (fig. 48, a), two long subulate tentacles b, with the eyes c usually at some distance from the base; foot d long and broad, truncate in front; mantle with side-lobes which can be reflected over the shell. These lobes, in most species, are beset with tentacles, often branched. The shell is richly enamelled by the secretion of the palatal lobes. It is convolute, with the spire concealed in the adult; the aperture narrow, enamelled at each end; the outer lip, in the adult, thickened and inflated; often transversely crenate; no operculum. In the young of Cypraea the shell has a thin and sharp outer lip, a short but conspicuous spire, and a thin periostracum; the spire becomes concealed by the subsequent enamel deposits from the reflected mantle-lobes; the line along which these meet at the back of the skull is usually indicated by a pale line on the shell.

Sp. Cypraea tigris (Tiger Cowry) (fig. 48).

Sp. Cypraea moneta (Money Cowry).—Of this little shell many tons' weight are annually imported into England, for the purpose of barter with the negroes of the west coast of Africa.

Genus Ovulum, Brug.—This group of cowries has been separated from Cypraea proper on some characters of the shell which are barely of sub-generic value. In the Ovulum edolus, or "weaver's shuttle shell," the mantle is furnished near the edge with a row of dark tubercles. A living specimen was taken by Mr Adams from a rocky coral bottom off the Island of Basilan, in the Mindoro Sea. It was languid in its movements, with the foot narrow, and folded longitudinally upon the slender coral branches along which it slowly glided.

The Ovulum verrucosum (fig. 81) approaches nearer to the type of Cypraea, having the mantle h partially lobed on each side; but it does not wholly cover the shell. The foot dd is flat and expanded; it is spotted black, on a white ground like the mantle. The tentacles b are subulate, of a pure white, with black extremities; the eyes are on the outer side of their base. The longest slope and narrowest end is the fore-part of the shell.

This species is a sluggish, slow-moving Mollusk, not more sensible to alarm than the true cowries.

The specimen figured was captured at Mindoro, one of the Philippine Islands, on a sandy bottom, by Mr Adams.

Family II.—Volutidae, Woodward.

Head with tentacles supporting eyes near their base; foot very large, partly covering the shell when out of water; mantle in some genera reflected upon the shell; siphon recurved. The shell turretted or convolute; aperture notched in front; columella obliquely plaited; no operculum.

Genus Marginella.—In this genus, as in Cypraea, the outer lip of the shell becomes thickened at the border (fig. 82), the spire becomes nearly or quite concealed, and the outer surface is enamelled.

Sp. Marginella undulata, Deshayes (fig. 82).—This species was taken alive by Mr Adams from a sandy bottom in the east coast of Africa. The proboscis a, tentacles b, the siphon, the foot d, and the mantle, are mottled with carmine on a yellowish ground, the border of the mantle bearing large crimson spots.

In the Marginella diadocbus, Ad., the tentacles are yellowish with a row of marble crimsoned spots, the eyes black and minute, the mantle pale, semi-transparent, of a pinkish yellow, with a row of semioval crimson spots round the thin free edge, the remainder being covered with radiating linear spots and short waved lines of a crimson colour; siphon marbled with crimson; foot of a delicate yellowish-pink, marked with deep crimson rays. The shell is of a bright olive carnelian hue, conspicuously encircled at irregular intervals with broad black lines, having almost the consistency of bands. This species was captured in the Straits of Sunda, from a sandy bottom, at a depth of about three fathoms.

The tentacles of the Marginella appear to vary in different species. In those observed by M. Deshayes on the shores of the Mediterranean the tentacles are described as being short, whilst in this and the preceding species they are slender and elongated. The eyes of the Marginella diadema are more pedunculated than those of M. undulata (Adams).

Genus Voluta, Linn., Lam.—In this genus the siphon has a lobe on each side near the base; the shell is ventricose, thick, with a short spire, having a mammillated apex. The aperture is large, deeply notched in front; columella plaited.

A beautiful species (Voluta abyssicola) was discovered by Mr Adams at the Cape of Good Hope.

"This elaborately carved species is one of considerable interest in a geological point of view, from the circumstance of its being the first living representative yet discovered of a group of highly sculptured Volutes abounding in the Eocene portion of the Tertiary beds of the British Isles. The principal of these, V. lima, elevata, crenulata, and digitalina, were distinguished by Mr Swainson as a sub-genus, under the name Volutilithes. The Voluta abyssicola is not identical in species with the fossils, being characterized by a closer and more sharply-defined pattern of lattice-work, which comprises as many as thirty transverse, and forty longitudinal ridges in a whirl. The upper edge of the whorls is depressly flattened at the sutures, forming a narrow ascending canal. The ridges are slightly nodulous at the point of crossing, and round the upper extremity impart a coronated aspect to the shell. The columellar plaits, four in number, are sharp and delicate. The outer lip is thin, and does not appear to be mature. The only specimen collected was dredged from a bank of dead shells and rounded iron-stones, at the depth of 132 fathoms." (Adams.)

Genus Mitra, Lam. (Mitre Shells).—The animal of Mitra has in general a very short foot, straight and continuous from side to side in some species, but in others notched and produced, with a thickened anterior margin. It is commonly narrow and rounded, or acuminate posteriorly, and it bears a very small semi-transparent horny operculum, in some instances scarcely visible. The siphon is mostly directed forwards, and the somewhat short tapering tentacles have the eyes either situated about half-way, or they are placed on the outer side of the base. The head is long and very flat, and the tentacles are very close together at their bases. The proboscis is rarely exserted when they are crawling and lively, but as they become languid after capture it becomes distended with water and protrudes considerably. The animal of Mitra flamigera, one of the deep-water species, is very prettily marked. The body is gray, varied with round, well-defined white spots, and dark-brown blotches, of a pyramidal form, arranged in a row round the lower edge in a Vandyke pattern, and below that a white rim with a row of small, linear, horizontal black spots; the head is white, marbled with gray-brown; the eyes black, and the tentacles white, with a large oval black spot in their middle; the siphon is brown, edged with black, and with a broad white band at its free extremity. The operculum is very minute, horny, and transparent. Hab. Caramata Passage; 14 fathoms, hard muddy bottom, mixed with sand and broken shells.

Another species, with the same habits, the Mitra interlirata, is semiopaque, white, faintly mottled with light brown, with the eyes at the outer base of the tentacles, and black. Hab. China Sea; 10 fathoms.

"The animal of that division of the genus which Swainson included under Conohelix is the same as in the typical species. I have found the Mitra conus buried rather deep in the soft black mud under the roots of trees in mangrove swamps, above high-water mark, in the Island of Basilan. The M. conica is found in company with other species of mitres, crawling slowly over the sandy mud in shallow places, among the islands of the Philippine group.

"Although M. Quoy has rightly termed the Mitra an 'animale apathique,' I have seen the small longitudinally-ribbed species crawl about pretty briskly over the smooth sand among the low coral islands. The Mitra episcopalis, probably on account of the small size of its locomotive disc, and the ponderous nature of its long shell, is a very sluggish Mollusk. I have observed some of the Auricularia-shaped mitres that live among the Philippines, in the shallow pools left by the receding tide, crawling about the stones out of the water, in company with Planaxis and Quoyia. The Mitres, like many of the large Volutes, prefer, however, to associate together, and may be seen in dozens crawling over the sandy mud-flats in shallow water, being most active just as the flood-tide makes. When the tide recedes, they bury themselves superficially in the yielding soil, and are with difficulty discovered. Some of the small-ribbed species cover themselves entirely with the sandy mud, and in that disguised condition travel about with comparative security. On one occasion, on the small island of Ambolan, at the south end of Mindoro, I was walking up to my ankles over a firm sandy mud-flat, taking little notice of the Conus, Strombia, Melongena, and Volutes, which people the water in great numbers, but looking about anxiously for the rarer mitres, when I first perceived these small species, under their ingenious disguise, marching in towards the shore as the tide flowed rapidly over the level surface. Persons, by the way, should never venture in places of this description barefooted, as there is a species of Pinna which buries its sharp end in the mud, but leaves the thin trenchant edges of the gaping..." extremity exposed, and when trodden on, inflicts very deep and painful incised wounds. Both myself and several of the boat's crew suffered in this way."

"The Philippine Islands would seem to harbour the greatest number of these elegant and beautiful shells, although a great many species were obtained by Mr. Cuming in tropical America. They appear to be chiefly confined to the equatorial regions, scarcely any being natives of cold climates. I have met with several among the Mako-shima Islands, at Loo-Choo, at Japan, and at the Keeling or Cocos Islands. They are most frequently to be met with in somewhat shallow water among the ledges of rocks, between small islands where the water barely covers the land, and within the shelter of coral reefs; sometimes preferring a clear sandy bottom, and sometimes affecting a hard muddy sandy soil. The transversely ribbed species are frequently found in very deep water, and many were dredged by us in 20 and 30 fathoms at Sooloo and in the China Sea." (Adams.)

**Family III.—BUCCINIDÆ.**

*(Whelks and their allies.)*

Branchiae two. Predatory on other Mollusks.

**Genus Ancillaria**, Lam.—"The animal of *Ancillaria* (fig. 83) is voluminous, covering the entire shell, with the exception of the spire. The head, which is entirely concealed by the reflected portions of the foot, consists of a short, inflated, cylindrical, annulated proboscis, above which is a semilunar veil formed by the dilatation and union of the tentacles; there is no indication of eyes. The mantle lines the shell, and is produced anteriorly into a long siphon. The foot is large and bursiform, the side-edges being greatly extended and reflected over the shell, meeting in the middle on the back. As in *Oliva*, it is deeply fissured anteriorly, forming a semilunar disc before the head, divided by a deep longitudinal groove into two lateral, triangular lobes, acuminate transversely; posteriorly it is bilobed, and is either without an operculum, or is provided with a thin, horny, unguiform one, with apical nucleus, semilunar striae, and an oval muscular impression.

"Sp. *Ancillaria obtusa* (fig. 83) (Swainson, *Journ. Sci. Lit.* and Arts, vol. xviii., p. 282; Sowerby, species *Conchiliorum*, *Ane.*, p. 5, fig. 24, 25).—Hab. east coast of Africa, below Port Natal. The specimen taken alive at the above-named locality was of a dirty white colour, marked with dull brown elongated blotches, distributed with scarcely so much regularity as represented in our figure. The operculum is shown above the chief figure.

"The *Ancillaria* resemble the *Oliva* in their habits, dwelling among the smooth sands in which they frequently bury themselves. They crawl with a quick sliding motion, and, as they glide briskly along, the tubular cylindrical siphon only is visible, directed backwards and upwards, and even laid flat upon the back; the alar expansions of the foot slightly overlap each other in the middle, and, extending considerably beyond the spire, form posteriorly a loose open sac. It is possible that the dilated lobes of the foot are sometimes extended and serve for swimming, as D'Orbigny has observed in *Oliva Tehuelchana* (*Voy. Amer. Mérid.* Moll., p. 419, A. A.)." (Adams.)

**Genus Dolium**, Lam. (Tunas).—Head with a long proboscis (fig. 49, a); tentacles & subulate, with the eyes at the outer side of their base; siphon slender, canaliculate; gastero-intromittent organ y in the male very large, grooved, bent, with a plicate glans; foot d produced in front of the head without an operculum; pallial border i entire. Shell ventricose, spirally furrowed; spire small; aperture very large; outer lip crenate.

Sp. *Dolium galea* (Helmet-Tun), fig. 49.—Shell and soft parts. About fourteen species have been characterized. The helmet-tun is a native of the Mediterranean; but most of the species are from warmer seas, as those of Ceylon, China, Australia, and Polynesia. They affect reefs and rocky beds, and some of the species acquire a great size.

**Genus Columbella**, Lam.—The animal of *Columbella* has a long and somewhat narrow vertically depressed head, with the eyes sometimes placed on the outer side of the base of the tentacles, and sometimes on the outer side of reflected prominences, situated at some little distance from the head. The siphon, long and directed forwards, is considerably dilated at the anterior extremity. The foot is short and pointed posteriorly, and bears a small, semi-transparent, horny operculum, with concentric elements. Anteriorly the foot is often considerably produced beyond the head, where it forms a long, thick, flattened, fleshy, finger-like process. Sometimes it is expanded laterally, when it is truncate anteriorly and furnished with two lateral angular processes.

Sp. *Columbella teniata* (fig. 84).—Hab. Borneo.

There are two fillets of square red-brown spots on each whirl, the lower of which is concealed in all but the last whirl. The Columbelle live in shallow water, on sandy flats, or congregating about stones.

**Genus Harpa**, Lam. (Harp-Shells).—The animal of *Harpa* has a very large foot (fig. 85, d), semicircular anteriorly, divided by lateral fissures from the posterior part e, which does not bear an operculum, and is said to be spontaneously detached when the animal is molested; tentacles b conical, with the eyes at the outer side of the base; siphon h cylindrical, elongate. Shell ventricose, with ribs at regular intervals; spire short; aperture large, notched in front.

Sp. *Harpa ventrosa*, Lam. (*Buccinum Harpa*, Linn.)—Hab. Mauritius. The harp-shells are natives of tropical zones, occurring in the Indian and Pacific Oceans. They are generally brought up from deep water and soft or sandy beds.

**Genus Buccinum**, Lam. (Whelks).—The whelks have a lamellar operculum, with the nucleus external, or subcentral within the outer margin. Shell ovate; spire moderate; mouth oblong; outer lip rather sinuous; pillar... rounded; operculum ovate. The silicious teeth of the long tooth-strap are arranged in transverse rows of three, the central or "rachidian" tooth having seven points; the lateral teeth hook-shaped, with the scape tridentate. The animals of *Buccinum undatum* and *Chrysodomus antiquus* are both vended for food in the streets of London under the name of "whelks." They are also dredged for bait. The nidamental capsules of the *Buccinum undatum* are figured in cut 92. (For the anatomy of the whelk see Cuvier, *Ann. du Mus.* xi., pp.447–457; *Mém. s. l. Mollusq.*, No. 17.)

**Genus Eburna**, Lam. (Ivory-Shells).—An instructive drawing was made of the living *Eburna areolata*, Lamarck, during the voyage of the Samarang. It is described as follows by Mr Adams:

"Sp. *Eburna areolata*, Lam. (fig. 86) (*Eburna tesselata*, Swainson).—Head flat, extended; tentacles very long and slender; eyes consisting of a yellow iris and black pupil, mounted upon pedunculated swellings on the outer base of the tentacles; siphon large, fleshy, and slightly curved; foot long, fleshy, and robust, acuminate behind and carrying a horny operculum. Colour dull pinkish-white, sprinkled with large, light brown, irregular blotches; siphon and tentacles mottled with spots of the same colour." (Adams.)

The shells of the *Eburna* are dense and smooth, having usually lost their periostracum. When recent they bear dark-red spots on a pure white ground. They have been dredged from fourteen fathoms, and are pretty widely distributed; occurring at the Cape of Good Hope, the Red Sea, the Indian Ocean, Japan, China, and Australia.

**Family IV.—Muricidae**, Woodward.

Animal with a broad foot, much expanded in some; eyes sessile on the tentacles at or near their base; branchiae two. Shell with a straight anterior canal; aperture entire behind.

**Genus Murex**, Linn. (Rock-Shells).—"Shell ornamented with three or more continuous varices; aperture rounded."

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**Fig. 87.**

*Ficula lacivagata.*

**Fig. 86.**

*Eburna areolata.*

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**Genus Pyrula**, Lam. (*Ficula*, Swainson, Fig-Shells).—Head (fig. 87, C) elongated, slender, flattened; tentacles long, subulate, placed at the sides of the front, separated by a wide interval at their base; eyes large, black, and sessile on the outer side of the base of the tentacles; siphon elongated, subcylindrical, and produced; mantle thin and membranous, produced on each side into a rounded lobe, equally reflexed on each side over the shell; foot large, expanded, rounded in front, rather produced on each side of the front edge, expanded, broad and tapering, and not furnished with any operculum.

Sp. *Ficula lacivagata* (fig. 87), (Reeve, Conch. Icon., *Ficula*, pl. i., fig. 4; *Bulla ficus*, Linn.; *Pyrula ficus*, Lamarck).—Head and neck pink, varied with scattered yellow spots; mantle bright pink, mottled with white and darker pink; under surface of foot dark purple chocolate, varied with yellow scattered spots. Hab. Sookoo Sea, at the depth of 35 fathoms. The dark chocolate colouring of the under surface of the foot presents a rich contrast with the bright freckled pink of the upper.

Sp. *Ficula reticulata* (Reeve, Conch. Icon., *Ficula*, pl. i., fig. 1; *Pyrula reticulata*, Lamarck).—Head and tentacles white; mantle light pink, marbled and reticulated with darker pink; foot pink, with six large opaque white spots at about equal distances. Hab. west coast of Borneo; from mud at a depth of about 17 fathoms. The head of this species differs from that of the former in being colourless. The mantle is characterized by the same pink reticulated marbling as the foot.

"The *Ficula* is a very lively animal when observed in its..." native element, crawling along with considerable velocity, and, owing probably to the lightness of its shell, able to ascend the sides of a glass vessel, in which I had it captive, with facility. The proboscis is rarely exerted when the animal is in motion, but the long slender tentacles are stretched out to their full extent." (Adams.)

**Family V.—STROMBIDÆ**, Woodward.

Animal (fig. 88) with pedunculate eyes e, and tentacles b, from a common elongated base; foot d narrow and small, but produced, and subservient to active leaping movements, during which the shell oscillates from side to side; operculum f narrow, pointed, and serrated on the outer edge.

**Genus Strombus**, Linn. (*Wing-Shells*).—Shell subventricose; spire short; outer lip dilated into a very ample wing excised towards the canal by a sinus; canal straight, or inflected by the columella obliquely outwards; canal short, emarginate, or truncated; sinus of outer lip distinct from canal; in older shells this lip is expanded into a wing, simple and entire. *Strombus*, Lam.

Sp. *Strombus gigas*, L. (Lister, *Conch.*, tab. 863, fig. 18).—Hab. West Indies. This species forms the largest known univalve shell, weighing sometimes four or five pounds; its apex and spines become solidified by calcareous deposits with age. From its occasional use as a garden ornament it has been called "fountain-shell." Vast numbers are annually imported from the Bahamas for the manufacture of cameos and for porcelain works. Canal elongate; sinus of outer lip not contiguous to canal; the lip produced into digitate laciniae. *Pterocera*, Lam.

Sp. *Strombus Chiragra*, L. (Lister, *Conch.*, pl. 870, fig. 24); *Strombus scorpions*, L. (D'Argen., *Conch.*, pl. 14, fig. n.)

**Genus Rostellaria**, Lam. (fig. 88).—Shell with a long spire; whorls numerous; canal subulate, extending behind up the spire; sinus of outer lip contiguous to the canal. The Eocene fossil species have the outer lip enormously expanded.

Sp. *Rostellaria rectirostris*.—The subcylindrical annular proboscis (fig. 88, a) has a broad central dark-bronze line, the edges of which are yellow, bordered with vermilion; eyes c deep blue, with black pupils, surmounted on long cylindrical peduncles; tentacles b white, with a narrow vermilion streak along their anterior surface; body cylindrical and much elongated, marked with red-brown on the outer surface, white beneath; foot d narrow, rather dilated and rounded in front, with a thickened anterior margin, small and subquadrate behind, separated by a deep notch from the lid-bearing part e; operculum f ovate-triangular, annular, horny, semitransparent.

The *R. rectirostris*, like the rest of the *Strombidae*, progresses by bending the foot under the shell and suddenly straightening, which enables it to roll and leap over and over. (Adams, loc. cit.)

**Genus Terebellum**, Klein, Lam.—The discovery of the living *Terebellum* has occasioned the removal of that genus to this family, on account of its affinity with *Strombus*. The eyes are pedunculated, and the mantle is characterized by the same peculiar divided edge. In the narrow form of the foot and proboscis-like head it is allied to *Struthiolaria* and *Aporrhais*, and, like *Oliva*, the mantle has a long filamentary cord winding into the sutures of the shell.

Sp. *Terebellum subulatum* (fig. 89), Lamarck (*Anim. sans vert.*, Deshayes' edit., vol. x., p. 584).—Hab. China and Sooloo Archipelago.

"The animal of *Terebellum* may be thus described: Head proboscidiform C; tentacles connate with the long cylindrical eye-peduncles, at the ends of which are placed the eyes b, b; mantle with the right edge reflexed over the outer lip, produced in front into a short siphon a, and furnished behind with three or four filaments, the inner edge spread over the columella, and ending behind in a long slender filament, which occupies, as in *Oliva*, the channelled suture of the spire; foot large, ovate, fleshy, laterally compressed, with a lobe at the fore part, rounded behind, and bearing a minute, horny, triangular operculum f.

"The eye-peduncles of this species are finely dotted with brown, the proboscis and the fore part of the body is punctuated with the same; the rest of the body is opake white, with three large irregularly-shaped red-brown blotches on the fore part; the under surface of the foot is light brown, with a white subcruciate marking.

"The *Terebellum* is extremely shy in its movements. Poising its shell in a vertical position, and cautiously protruding its longest telescope-eye from the truncature in the front of the shell, it will remain stationary until assured of security. It will then use its pointed foot as a lever and roll its shell over and over, progressing by a series of irregular leaps. When removed from the water, before dying, it will jump several inches from the ground. Mr. Cuming assures me his knowledge of the animal coincides with my own experience, and that on one occasion he lost a fine specimen owing to its suddenly leaping from his hand into the water. I have observed both the varieties of this species alive. In the spotted variety, the muzzle is reddish towards the tip, the body is opake pearly white, the eye-peduncles mottled with dark red; in the common variety there are three large red-brown blotches on the fore part of the body." (Adams.)

With these active forms of carnivorous Gastropods, we seem to reach the summit of this branch of the great Molluscous tree.

The ova of the marine Gastropods are enveloped, before exclusion, in mucous capsules, prepared by a special gland situated near the termination of the oviduct. The secretion in some species is soft, flexible, and transparent; in most it hardens by contact with the sea-water, and assumes various definite and characteristic forms; the nidus is sometimes simple, sometimes compound, but each compartment contains many ova; and the development of the embryo proceeds in the nidamental chamber until its own little defensive shell is acquired.

In the terrestrial Gastropods the ova are usually spherical and opaque, and separately extruded: snails and slugs oviposit in the earth. The tropical Bulini cement leaves together to form an artificial nest for their large eggs.

Fig. 90 is an outline of the egg of the Bulinus ovatus of the natural size. The shell of the embryo is an inch in length when it is excluded. Specimens of both egg and young of the species are preserved in the Cumingian collection.

The ova of the sea-slug (Tritonia) are expelled together in the form of a long thread, and are arranged in a spiral manner in the tenacious transparent covering of the thread. In the Doris muricata the ova are aggregated in a flattened spirally disposed albuminous band when excluded from the oviduct.

The harder albuminous capsules which defend the ova of other marine Gastropods offer a great variety of forms, some of which are remarkable for their complexity, others for their symmetry and beauty. The nidamental sacs of the frail Janthina (fig. 91, b) are of a flattened pyriform shape, composed of a delicate reticulate film of albumen, and are attached by one extremity to a float a, formed likewise by a secretion of albuminous matter, dilated into a discoid group of cells filled with air. To this float the parent Janthina commits her little progeny, and having securely fastened their several cradles or nursery cells she detaches the float, which bears the ova to the surface, and sustains them where they may best receive the full influence of solar light and heat. The nidamental capsules of the Pyrula rapa are attached in regular linear series to portions of decayed wood; they are of a flattened sub-conical figure, adhere by their apex, and have their base emarginate. The nidamental capsules of the whelk (fig. 92), are common objects on our seashore; they are aggregated in large irregular masses, often attached to portions of oyster-shell; each capsule presents a depressed ovoid figure, with one side convex, the other flat or concave. The upper figure opposite the left hand shows the round hole in the inner side of the capsule, whence the young has escaped; the figure to the right is the shell of the embryo magnified four diameters. (Woodward). The small nidamental cells of the cowry (Cypraea) are aggregated in a flattened group. In the Turbinella the cells are of a flattened subpentagonal form, and adhere together, superimposed one upon the other, forming what is termed a camerated nidus. Each chamber contains between twenty and thirty embryos. The rudimental shell is completely calcified and fitted to defend the little Gastropod before it emerges from the temporary shelter provided for it by the parent. Numerous other modifications of these secreted nests of the Gastropodous Mollusca might be enumerated.

The development of the Gastropods has hitherto been traced in a few amoeatal, nudibranchiate, tectibranchiate, pulmonate, and pectinibranchiate species; and the results show considerable modifications in its course and phenomena. Most Gastropods are oviparous; some species of Litorina are ovo-viviparous; Paludina and Clausilia ventricosa are viviparous.

With the singular exception observed in Buccinum, in which the segmental or granular germ-mass from which the embryo is developed results from a confluence of numerous, perhaps fifty, previously distinct germ-masses in the nidamental capsule, that germ-mass is due to progressive segmentation of the yolk, the result of the usual multiplication of germ-cells, which clothe themselves with the so subdivided but coherent yolk-substance. One cell in Pulmonata and Aplysia, indeed, is from the beginning conspicuously larger than the rest, and has been called the directive cell (richtungs-bläschen).

In all Gastropods the germ-mass takes the form of a large round embryo, one end of which becomes indented and clothed by ciliated epithelium, by which it rotates on its axis in the albumen of the egg. Sars, who noted the oviposition of the Tritonia Ascana on the 1st of February 1840, traced the segmentation of the yolk to the 8-fold division on the 4th day, and the completion of the germ-mass on the 8th day. The indentation producing the bilobed end of the embryo took place on the 15th day; after which the ciliated lobes extended outwards, and assumed the form of wings or vela (fig. 93, c, e). Dr Grant had long before discovered the corresponding ciliated vela in the embryos of Purpura, Trochus, Nerita, Doris, and Eolis.

On the 20th day the rudiment of the foot appeared beneath the bases of the vela, and on the 22d a transparent shell s was developed, which covered all the body save the ciliated vela and foot. "I could scarce believe my eyes," writes Sars, "when I made this discovery." As the body and shell enlarge and elongate, the cilia become stronger on the lobes, and the active movements of the embryos, crossing each other like midges in the clear albumen of the nidamental band, offer a most singular spectacle; the vela now move by muscular contraction, alternately approximating and stretching outwards. On the hinder part of the foot is an operculum (fig. 93, f), which closes the mouth of the shell when the embryo retracts itself. Among the internal organs the acoustic capsules appear first, then the eyes. The tentacles next protrude, and the border of the mantle appears. The mouth is established between the vela. On the 36th day the stomach and a looped portion of intestine come into view out of the germ-mass, the remains of which is chiefly changed in the hepatic and genital glands. A longitudinal muscle for retracting the body into the shell also now appears. During this course of development the nidamental band has become, by endosmose of sea-water, three times as thick as before. The albuminous substance is absorbed by the embryos; they respire by the reaction of their ciliated surface on the imbibed water. As they

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1 See Prep. 2943 B., Hunterian Physiological Series, Lond. Coll. of Surgeons. 2 Ib., Prep. 2947 A. 3 Ib., Preps. 2948 and 2949. grow they with difficulty find room for their revolutions, and between the 32d and 38th days rupture the delicate membrane of their nest, and struggle out. They are now about one-eighth of a line in length, and swim by ciliary action, the vela being kept stiffly outstretched. They survived in the vessels of daily renewed sea-water two weeks, then died, their embryo shells floating on the surface.

The ova of the *Aplysia* are excluded in a long string, enveloped by a transparent flexible mucous, in the centre of which they are aggregated in several irregular series. When examined at this period, the yolk has apparently divided itself into six, seven, or more numerous globules, or, in other words, as many germinal vesicles, included in the same mass of albumen and in a common chorionic coat have given origin to as many aggregations of vitelline cells. These, therefore, may be regarded as so many independent yolks, in each of which the same progressive fissiparous multiplications have been observed as in the single vitellus of the ovum of the *Planorbis* and of animals in general.

After the multiplication of the globules has gone on to a certain point, two of them, of larger size than the rest, indicate, one the seat of the future branchial organs, the other that of the muscular mass.

The ciliated epithelium, with which the vitellus is now almost entirely covered, occasions the usual rotations of that body. The process of transformation of this monad-like embryo to the gastropodous form resembles closely that which has been described in *Tritonia*. The remains of the vitelline mass not yet metamorphosed into special organs indicate the expanded alimentary sac. The rudimental foot, and the velum with its strongly ciliated border, protrude from a rudimental, thin, pellicular, and flexible shell, which covers all the rest of the surface of the body. The embryo describes elliptical revolutions in the chorionic cavity. As development proceeds and the embryo increases in size, the shell acquires a more distinctly turbinated form, and is slightly bent out of its vertical plane. An operculum is formed upon the protruded surface of the foot. The course of internal development accords with that in *Tritonia*. The ciliated branchial surface begins to be withdrawn more into the interior; and in this state, protected completely by an external shell, the young *Aplysia* is launched into the ocean.

Truly may the subsequent growth, which effects an entirely internal position of the shell, with such a mutation of its form that the primitive nucleus can scarcely be detected upon the almost flattened plate, now destined to protect the equally internal respiratory organs of the mature animal,—justify us in applying to it the term "metamorphosis." This term is still more applicable to the developmental phenomena in the *Tritonia* and *Doris*, since these Gastropods, which are not only naked like the *Aplysia*, but are devoid of any internal rudiment of a shell, are yet provided with a delicate, little, operculated, nautiloid, horny, external shell, in their young state. The same general course of development in which the embryo or larval Mollusk is provided with the ciliated lobes and operculated shell, has been traced by Lovén in *Eoits*, *Bulla*, *Cerithium*; by Lund in *Murce* and *Natica*; by Nordmann in *Tergipes*; by Allman and Vogt in *Acteon*; by Carus in Pteropods; and by Siebold in *Vermetus*. Rudiments of the vela are retained in *Tergipes*, *Eolis*, *Doris*, *Tritonia*, and *Aplysia*; and in *Thetys* they continue in almost their primitive form and proportions, unless the broad head-lobes of the adult be substituted for the embryonal vela, as is the case with the fins of the Pteropods; otherwise the little *Cybulia*, with its delicate, symmetrical shell (fig. 89), would represent a persistent embryo form of the higher Gastropodous *Encephala*.

Professor Müller has detected ova and embryos of a Cephalopod, which he believes to belong to a species of *Natica*, within the body of the *Synapta digitata*. They were contained in elongated sacs, firmly attached or fused at one end to the head, at the other end to the gut, of the *Synapta*. The upper portion of the sac contains both spermatozoa (like those of *Natica*) and ova; the lower portion of the sac was intus-suscepted with a blind end, and this contained the ova, with developed embryos, according to the veiled type. This remarkable discovery indicates some singular parasitic habit in the generative economy of the Mollusk.

The development of the pulmonated Gastropods proceeds without any such metamorphosis as that above described. In the testaceous species its course has been already traced by Prevost and Dumortier in *Limnea*, by Pfeiffer in *Helix*, by Jacquemin and Quatrefages in *Planorbis*, and by Oscar Schmidt and Gegenbauer in *Helix*, *Clausilia*, and *Limax*.

The whole yolk is transmuted into a germ-mass. This becomes partially divided into a pallial or visceral and a pedial or somatal lobe. The fore part of the pallial portion dilates into a contractile sac, by Gegenbauer deemed the homologue of the "velum" of pectinibranchiate embryos. A similar contractile sac is formed at the end of the pedial lobe; and the contractions of the two sacs alternate, producing a flux and reflux of fluid pabulum before the heart is developed. A peculiar gland is attached to the pallial sac. The first granular rudiments of the shell are deposited in the substance of the mantle. The anterior tentacles and the parts about the mouth are the last to be completed.

**CLASS VI.—CEPHALOPODA**, Cuvier.

Encephalous Mollusks, with locomotive and prehensile organs radiating from the head; dioecious and amebianian. Animal divided into a somatal and pallial (fig. 94, m, d) portion. The former is chiefly muscular (fig. 94, f, h). It contains the organs of sense, mastication, and deglutition, and supports the organs of prehension and the chief powers of locomotion. It is called the "head" (fig. 97, a, e). The pallial division (fig. 97, a, b), termed "trunk," or abdomen, consists of a more or less muscular sac or mantle, with a transverse anterior aperture, from which an excretory siphon or "funnel" projects; and it contains the respiratory, generative, and digestive organs. The branchiae are pinnaflid and concealed. The sexes are distinct. All Cephalopods are oviparous. As far as observation has extended, a part only of the yolk is assimilated into a germ-mass; and development is progressive, without metamorphosis, to the completion of the miniature Cephalopod in egg.

**ORDER I.—TETRABRANCHIATA**, Owen.

Branchiae in two pairs, without branchial hearts; funnel formed by a convolute muscular plate; mantle thin, and feebly muscular; no ink-bag; arms very numerous, hollow, and with retractile tentacula; mandibles with calcareous tips; eyes pedunculate; head retractile, within a shell, which is external, many-chambered, siphunculate, the outer layers porcellaneous, the inner layers and partitions nacreous.

**Genus Nautilus**, Linn.—Shell discoid, symmetrical, with the apertures, sutures, and siphuncle, simple. The anatomical characters of the order are also those of the sole existing genus. It is the representative of numerous genera and species of chambered Cephalopods that abounded in the Palaeozoic and Secondary periods, but which seem to

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1 See Kölliker's masterly Entwicklungsgeschichte der Cephalopoden, 4to, 1844. have been superseded, as carnivorous Mollusks, in the Tertiary and recent periods, by the Pectinibranchiate Gastropods.

The organization of the Pearly Nautilus (Nautilus pompilius), first made known in 1832, throws light upon that of the extinct Ammonites, Orthoceratites, Lituites, Turritelles, &c., and possesses, therefore, an extrinsic interest, besides that which arises from the peculiar modifications of Molluscous structure which it presents. The soft parts (fig. 94, t, o) form an oblong mass, divided by a constriction into two slightly unequal segments; the posterior (m to o) is smoothly rounded, soft, and membranous, containing the viscera, and adapted to the last chamber of the shell; the anterior (m to t) is densely muscular, and includes the organs of sense and locomotion. It can be retracted within that chamber, as is shown in fig. 94, which is here selected as


showing the relations of the soft parts of the Pearly Nautilus to the shell in a specimen in which those relations had never been disturbed. The subject dissected and described by Professor Owen in 1832 consisted of the soft parts only, and their relations to the shell were determined on anatomical grounds, as they are illustrated in plate 1 of the Memoir above cited. The author's deductions were contested by Dr Gray, in the Philosophical Transactions for 1833, p. 774; by Professor R. Grant, in the Lancet, 1833, pp. 506 and 509; and by M. de Blainville, in the Nouvelles Annales du Muséum, tom. iii., p. 7. The second specimen of the Pearly Nautilus, which was brought to Europe in 1838, consisted also of the soft parts only. It was described by Professor Valenciennes, who adopted Professor Owen's conclusions as to their relative position to the shell. The third specimen, brought to England by Captain Sir Edward Belcher, R.N., in 1842, was contained in the shell, but not attached. It had been taken out, and been replaced in the position it originally occupied, according to Sir E. Belcher's convictions. That position agreed with the figure in plate 1 of Professor Owen's Memoir. The present figure, from a drawing in the possession of Dr Gray, F.R.S., is from a specimen in the British Museum, naturally attached to and retracted within the shell, a portion of which has been removed to show the soft parts; and the principal viscera are indicated in outline.

The mantle is very thin upon the posterior part of the body. It is continued backwards in the form of a slender tube, which penetrates the calcareous siphon in the septum closing the occupied chamber behind, and is thence continued, as the membranous siphon, through all the other divisions of the shell to the central nucleus. As the mantle advances towards the anterior part of the abdomen it increases in thickness, becomes more muscular, extends freely outwards (m), and forms a wide concave fold in the dorsal aspect, which is reflected over the black-stained involuted convexity of the shell s. The margin or collar of the mantle is continued downwards and forwards on each side with a sinuous outline, and is perforated below for the passage of the muscular expiratory and excretory tube called the "funnel" f. In the female Nautilus the nidamental glands form two circular convexities on the ventral surface of the abdomen, behind which the mantle is encircled by a thin layer of brown matter, like the periostacrum, which is very narrow above and below, but expands on each side into a broad plate a, corresponding in size and form with the surfaces of attachment of the two great muscles for adhesion to the shell.

The somatal division forms a strong and wide sheath, containing the mouth and its more immediate appendages; its inner surface is for the most part smooth, the outer one divided and extended into many parts or processes. The chief of these forms a broad triangular muscular plate or hood h, covering the upper part of the head, and presenting a middle and two lateral supercies; the former being traversed by a median longitudinal furrow, indicating the place of confluence of the two large hollow tentaculiferous processes of which it is composed. The back part of the hood is excavated for the lodgment of the involuted convexity of the shell, and the above-described fold of the mantle s covering it. Each side of the head supports a group of perforated processes or digitations t, the largest of which is next the hood, and the rest decrease in size as they descend in position. Exclusive of the short subocular perforated process, and of the confluent pair forming the hood, the digitations are eighteen in number on each side, and are of a conical form; each contains a long and finely annulated tentacle, with the inner surface supporting narrow, close-set transverse plates.

To the nineteen tentacula which are supported by the confluent and free digitations on each side of the head, two others must be added, which project from very short sheaths, one before, the other behind, the eye; the lateral transverse incisions are deeper in these than in the digital tentacles. The eyes are about the size of hazel-nuts, and are attached each by a short peduncle to the side of the head, behind the digitations, and a little below the margin of the hood. The inferior surface of the oral sheath is excavated for the lodgment of the infundibulum f. It appears that, amongst other remarkable peculiarities of the Nautilus, is its possession of external ears. Mr Macdonald writes:—“Both Professors Owen and Valenciennes noticed that the hollow subocular process of their specimens of Nautilus pompilius was not tentaculiferous, and I may be permitted to say that this was also true of several specimens of Nautilus pompilius, and one of N. macromphalus examined by me. But there is still another matter worthy of remark with reference to this process, namely, that its cavity may be traced downwards, inwards, and a little forwards, to within about the

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1 Owen, Memoir on the Pearly Nautilus (Nautilus pompilius, Linn.), &c., 4to, 1832. 2 Nouvelles recherches sur la Nautilé flambé, Archives du Muséum, 4to, 1839. twentieth of an inch of the auditory capsule; indeed it would appear as though provision had been made for the entrance of sonorous waves through a rudimentary external ear." "The interior of the tube is lined with a glandular membrane, thrown into small folds disposed longitudinally."

The mouth is armed with two mandibles, shaped, as in other Cephalopods, like the beak of a parrot reversed, the lower mandible overlapping and curving upwards beyond the upper one. Both mandibles are horny, with their tips encased by dense calcareous matter, and their base implanted in the thick muscular parietes of the mouth.

They are immediately surrounded by a circular fleshy lip with a plicated anterior border, external to which there are four broad flattened processes continued forwards from the inner surface of the oral sheath, two of which are superior, posterior, and external, the other two are inferior, anterior, and more immediately embracing the mouth; the latter are connected together along their inferior margins by a middle lobe, the inner surface of which supports a series of longitudinal lamellae. On the inner surface of the oral sheath beneath these processes there are two clusters of soft conical papillae, and on each side of these a group of lamellae. Each of the four processes, which are called "labial," is pierced by twelve canals, the orifices of which project in the form of short tubular processes from the anterior margin, and each canal contains a tentacle similar to, but somewhat smaller than those of, the digitations. Thus the number of tentacula with which the Pearly Nautilus is provided, amounts to not less than ninety, of which thirty-eight may be termed digital, four ophthalmic, and forty-eight labial.

All Cephalopods have an internal cartilaginous skeleton: that of the Nautilus is confined to the inferior surface of the head; no part of it extends above the oesophagus. Viewed sideways, it presents a triangular form; a portion of the annular brain is protected by a groove on the upper surface of the cartilage; two strong processes are continued from its anterior and superior angles into the crura of the infundibulum, giving origin to the chief muscles of that part. Two other thinner processes are continued backwards, and curve inwards and downwards: they give origin to the two great muscles which pass from the internal to the external skeleton, or, in other words, attach the animal to the shell.

The muscular fibres of the "soma" or oral sheath arise from the whole of the anterior or outer part of the internal skeleton. The muscular structure of the funnel presents a much greater development than in the naked Cephalopods; and, from its relation to those masses which, on the one hand, attach the soft parts to the shell at α, and, on the other, connect the head to the trunk, we may conclude that the funnel is the principal organ of natation, and that the Nautilus is propelled, like the Octopus, by a succession of jerks occasioned by the re-action of the respiratory currents upon the surrounding water. The orifice of the funnel is guarded by a valve.

The principal masses of the nervous system are concentrated in the head, and are more or less inclosed by the cephalic cartilage. The superoesophageal portion is a thick transverse rounded chord, connected at each extremity with three ganglionic masses; the middle and superior of these (ophthalmic ganglia) supply the eyes; the anterior and inferior pair are united below the oesophagus; the posterior and inferior pair form a second oesophageal nervous centre. The nerves given off immediately from the super-oesophageal mass supply the muscular and other parts of the mouth, and have small buccal ganglia developed upon them. The anterior oesophageal ring gives off principally the nerves to the tentacula, and the two median ones are connected with a ganglion, which supplies the tentacula of the inferior labial processes and the lamellated organs on that part of the oral sheath. The tentacular nerves are continued, like those of the arms in the higher Cephalopods, along the middle of the tentacle, attached by loose cellular tissue to the vessels of the part. The posterior collar gives off numerous nerves of a flattened form, which supply the muscles of the shell. The respiratory nerves form a small ganglion at the base of each pair of gills, from which branches are sent to those organs, to the heart, and to the appendages of the veins. A plexus of more delicate visceral nerves is continued backward along the interspace of the branchial nerves, and the chief branches are connected with a small ganglion situated between the cardiac and pyloric orifices of the stomach.

The calcareous extremity of the upper mandible is sharp-pointed and solid to the extent of five lines. The lower mandible is sheathed with a thinner layer of the hard white substance, which forms a dentated margin. The fossils termed "rhyncholites" are the homologues of these calcareous extremities of the beak in cognate extinct Cephalopods. The muscular subspherical mass, which supports and moves the mandibles, is provided with four retractors, and can be protruded by a strong semi-circular muscle, which is continued from the margin of one of the inferior labial processes over the mandibles and their retractor muscles to the labial process of the opposite side.

The glottidium is supported by a horny, slightly curved, and transversely striated plate. The fleshy substance forms three distinct papillose caruncles anteriorly, or "tongue" proper, into which the retractor muscles are inserted. The tooth-strap supports four longitudinal rows of recurved spines, behind which the surface of the glottidium is again soft and papillose. Two broad duplicatures of mucous membrane project forwards from the sides of the pharynx; they each include a simple layer of salivary follicles, the secretion of which escapes by a single perforation in the middle of the process.

The lining membrane of the pharynx is disposed in numerous longitudinal folds, where it begins to contract into the oesophagus. This tube, having passed through the nervous collar, dilates into a capacious crop (fig. 94, c), from the bottom of which a contracted canal, half an inch in length, is continued to an oval gizzard g. The intestine commences near the cardiac orifice, and soon communicates with a small, round, laminated pouch, through which the biliary secretion passes to the intestine. This tube forms two abrupt inflections, and terminates in the branchial cavity near the base of the funnel close to the proboscidian end of the oviduct.

The epithelium of the oesophagus and ingluvies is developed into a thick cuticular membrane, with minute ridges in the gizzard. In the specimen dissected by the writer, the crop and gizzard were laden with the fragments of a small crab, the pieces being more comminuted in the gizzard.

The liver f is a bulky gland, extending on each side of the crop as low down as the gizzard; it is divided into four lobes, connected posteriorly by a fifth transverse portion: the lobes are subdivided into numerous lobules of an angular form. The secretion of the bile is derived, as in other Mollusks, from arterial blood; it is conveyed from the liver by two main trunks, which unite into one duct, about two lines from the laminated sac. The bile, having entered the sac, is diverted by a peculiar development and disposition of one of the laminae from flowing towards the gizzard. The follicular structure of this and the other folds of membrane indicate their glandular character; and the entire laminated pouch may be considered as a more developed form of pancreas than the simple cecum which represents that gland in some of the Gastropods. No other foreign

1 Proceedings of the Royal Society, 1856, p. 381. secretion enters the alimentary canal, as there is not any ink-gland in the Pearly Nautilus.

The heart and large vessels, with their follicular appendages, are contained in a large cavity opposite the letter h, subdivided into several compartments. Some of these contain groups of follicles developed from the veins proceeding to the branchiae, and they communicate by small apertures with the pallial cavity. Sea-water is thus admissible into the pericardium, and thence into the siphuncle. Any matter excreted from the venous follicles, which seem homologous with the so-called "renal gland" of lower Mollusks, may escape by the above apertures into the pallial cavity. The principal venous trunk, returning the blood from the soma, is separated from the abdominal cavity by a layer of decussating, chiefly transversely disposed, muscular fibres. There are several small intervals left between the muscular fibres and corresponding round apertures in the venous tunics. These communications with the general abdominal cavity are similar to that discovered by Cuvier in the Aplysia. M. Valenciennes detected the same structure in the specimen of the Nautilus dissected by him. The blood from the diffused abdominal venous sinus is thus received into the chief vein proceeding to the gills.

The branchial circulation may be considered to commence when the blood again begins to move from trunk to branches, four of which trunks are continued from the terminal venous sinus to convey the carbonized blood to the four gills, of which there is a larger and a smaller one on each side. Each pair of gills, situated opposite the letter b in fig. 94, is connected by a common peduncle to the inner surface of the mantle; the larger branchia consists of a central stem supporting forty-eight vascular plicated lamellae on each side; the smaller branchia has thirty-six similar lamellae on each side.

The four vessels continued from the venous sinus have attached to them, in their course to the gills, the clusters of glandular follicles above mentioned. The veins extend beyond the follicles each to the root of its respective gill, where it receives a small vein. At this part there is a valve which opposes the retrogression of the blood; the vessel, which may now be termed branchial artery, penetrates the root of the gill, and dilates into a wider canal, which is continued through the soft white substance forming the branchial stem. A double series of branches are sent off from the lateral lamellae, which ramify and subdivide to form the capillary plexus, from which the returning vessels terminate in the branchial vein. These veins quit the roots of the gills, and return to terminate at the four corners of a subquadrilateral transversely elongated ventricle. From this ventricle two arteries arise, one anterior, the other posterior, of large size, and with a muscular bulb at its commencement, with which is connected an elongated, pyriform, apparently closed sac.

The female organs of the Nautilus consist of an ovary, an oviduct, and, as in the Pectinibranchiate Gastropods, of an accessory glandular nidamental apparatus. The ovary is situated on the right side of the gizzard in a peritoneal cavity peculiar to itself. It is an oblong compressed body, one inch and a half in length, and an inch in breadth; convex towards the lateral aspect, and on the opposite side having two surfaces sloping away from a middle longitudinal elevation. At the anterior and dorsal angle there is an orifice about three lines in diameter, with a puckered margin, which conducts into the interior of the ovary. It is filled with numerous oval ovisacs of different sizes, which are attached by one extremity to the ovarian capsule, but are free and perforated at the opposite end. The oviduct terminates at the base of the funnel near the anus. The nidamental gland consists of numerous close-set pectinated lamellae. An interesting account of the dissection of a male Nautilus Pomplius has been contributed by Professor Van der Hoeven to the fourth volume of the London Zoological Transactions, p. 21. Professor Vrolk has published a good description and figures of a specimen in his collection, from Amboyna, in the tenth volume of the Mémoires de la Société Linnéenne de Normandie, 1855.

In contrasting the organization of the Nautilus with that of the inferior Mollusca, already treated of, we find the main advance to have been made in the organs of animal life. A true internal skeleton is established in the Nautilus, and thus the lowest Cephalopod offers an approximation to the vertebrate type, which not even the highest of the articulate series had attained. Perfect symmetry now reigns throughout the animal and vital organs. The muscular system forms a larger proportion of the body, with various arrangements and complications unknown in the lower Encephaulous Mollusks. The respiratory tube, though still completed by the overlapping, not by the coalescence, of its side-walls, has received an enormous development as contrasted with the siphonated Trachilipods; and, by its powerful muscles, and their firm cartilaginous basis of attachment, would seem to be endowed with a new function, in relation to propelling the Cephalopod with its testaceous dwelling through the sea.

The nervous centres concentrated in the head have received a marked increase of bulk, which, nevertheless, is still manifested more strongly in the inferior masses, and especially in the anterior suboesophageal ring than in the superior or cerebral part. Here, however, we find for the first time in the Molluscous series, especial ganglions subordinated to the greatly enlarged organs of vision.

The organs of reptation, which had progressively advanced (as Lamarck's denomination of the higher Gastropods indicates) towards the head, are exclusively attached to that part in the Nautilus, and project from before the eyes and mouth. The mouth, besides its jaws and spiny tongue, is now served by organs of prehension; and it is most interesting to observe that these cephalic, prehensile, as well as exploratory, tentacula, at their first appearance manifest the vegetative character in their multiplied repetition and comparative simplicity, compared with their homologues in the Dibranchiate Cephalopods.

Some of the Gastropods have a pair of jaws working upon each other, but in the horizontal plane, as in insects. In the Nautilus they are opposed to each other vertically, as in the vertebrate series, and they present a form which is repeated amongst fishes by the Scari, amongst reptiles in the Chelonia, and almost universally in the class of Birds. The close resemblance to the latter class which the Nautilus offers in the modifications of the alimentary canal is sufficiently striking, but hardly more so than some of the Bryozoa present, in which radiated animacules may be discerned one of the roots of the great Molluscous branch of the Animal Kingdom.

In the very few Conchiferous Gastropods that are able to swim the shell is of diminutive size, of a simple form and structure, and of an extremely light and delicate texture. The strong and muscular occupant of the Pearly Nautilus shell would seem to have that abode adapted for occasional natation by the air-chambers and siphuncle. The first living specimen which was captured and brought to Europe in a state fit for dissection was observed floating on the sea with the shell upwards, and was obtained by the boat-hook. The parts of the shell, progressively vacated during the growth of the animal, are successively partitioned off by smooth plates concave towards the outlet. The formation of these proceeds from the circumference to the centre, and there meeting with the siphuncular prolongation of the mantle, which retains its primitive connection with the shell-nucleus, calcification is continued backwards for a short distance around that process, which now forms the membranous siphon, and acquires the partial protection of the calcareous tube. An air-tight chamber is thus formed, traversed by the siphon; by a repetition of the same processes a second chamber is formed, included within two perforated septa; and similar, but wider partitions continue to be added, concurrently with the formation of the new layers which extend and expand the mouth of the shell, until the animal acquires its full growth, which is indicated by the body having receded for a less distance from the penultimate septum before the formation of the last septum is begun.

The periodical formation of these septa in the progress of growth is analogous to that of the projecting external plates in the Wendletrap, and of the rows of spines in the Murce; but these external processes consist of the opake calcareous layer of the shell, whilst the internal processes in the Nautilus consist of the nacreous layer like the septa in the Turritella. Thus the embryo Nautilus at first inhabits a simple shell like that of most univalve Mollusca, and manifests, according to the usual law, the most general type at the early stage of its existence; although it soon begins, and apparently before having quitted the ovum, to take on the special form.

In acquiring the camerate structure of the shell, the Nautilus gains the power of rising from the bottom, and the requisite condition for swimming; by the exhalation of some light gas into the deserted chambers, it attaches to its otherwise too heavy body a contrivance for ascending in its atmosphere, as we ascend in ours, by the aid of a balloon. But the Nautilus, superior to the human aeronaut, combines with the power of elevating and suspending itself in the aqueous medium, that of opposing its currents and propelling itself at will in any direction. It possesses the latter essential adjunct to the utility of the balloon as a locomotive organ, by virtue of the muscular funnel, through which it ejects into the surrounding water, doubtless with considerable force, the respiratory currents.

It appears that the proportion of the air-chambers to the dwelling-chamber of the Nautilus and its contents is such as to render it of nearly the same specific gravity as the surrounding water. The siphon, which traverses the air-chambers, communicates with the pericardium, and is most probably filled with fluid from that cavity. It certainly conduces small blood-vessels, which are essential to the maintenance of the vitality of the chambered part of the shell.

In air a large and perfect shell of the Nautilus Pompilius weighed six ounces and a-half avoirdupois; it required an additional weight of one ounce seven drams to sink it in water. The soft parts of a female Pearly Nautilus weighed, in air, five ounces; but the specific gravity, including the contents of the crop, was nearly that of sea-water.

The power by which the Nautilus alters its specific gravity is probably like that possessed by the fresh-water testaceous Gastropods, depending chiefly upon changes in the extent of the surface which the soft parts expose to the water, according as they may be expanded to the utmost, and spread abroad beyond the aperture of the shell, or be contracted into a dense mass within its cavity. The Nautilus would likewise possess the additional advantage of producing a slight vacuum in the posterior parts of the chamber of occupation which is shut out by the horny cincture and muscles of adhesion from the rest of that cavity; and it is possible that the gas in the last air-chamber might expand as the Nautilus protrudes from the shell, and be in the same degree condensed as it forcibly drew itself back.

Whatever additional advantage the existing Nautilus might derive, by the continuation of a vascular organized Cephalo-membranous siphon through the air-chambers, in relation to the maintenance of vital harmony between the soft and testaceous parts, such likewise must have been enjoyed by the numerous extinct species of the Tetrabranchiate Cephalopods, which, like the Nautilus, were lodged in chambered and siphoniferous shells.

Sp. Nautilus Pompilius (Pearly Nautilus).—This species ranges from the Persian Gulf and Indian Ocean, to the Chinese Seas and Pacific.

Sp. Nautilus scrobiculatus (Umbilicate Nautilus).—Hab. the warmer latitudes of the South Pacific.

Sp. Nautilus macromphalus (Widely Umbilicate Nautilus).—Hab. New Caledonia, and neighbouring isles of the South Pacific.

It is not certain to which of the above species the following remark, on their abundance, applies:—"The Pearly Nautilus is so abundant, that its shell serves the Papuans of Port Praslin in 'Novelle Bretagne,' and throughout the Papuan Archipelago, as a scoop to bale out the water from their canoes; the debris of the shell cover the shores of those islands."

If the Nautilus extended itself in a straight line during its growth, instead of revolving round an imaginary axis, a straight conical shell would be produced, with the chambered part divided by simple septa concave next the outlet. Such are, in fact, the characters of the fossil shells called Orthoceratites.

The margins of the septa of the shell in all the existing species of Nautilus are slightly sinuous, which makes the surface next the aperture of the shell convex at one part and concave at another. In an extensive genus of extinct chambered shells called Ammonites, the sinuosity of the margins of the septa is much greater, and most of the surface next the outlet is convex: the siphon perforates the septa at their centre in extremely few species, and in the rest is situated at that margin which is next the outer curve or circumference of the shell. Certain chambered shells thus characterized are straight, like the Orthoceratites, but generally compressed, with their numerous septa joining the outer shell by foliated dentations: they are termed Baculites. In the true Ammonites the shell is discoid, and coiled upon itself as in the Nautilus; but it is strengthened by arched ribs and dome-shaped elevations on the convex surface, and by the tortuous windings of the foliated margin of the transverse partitions. Separate casts of the interior of the chambers are not unfrequently obtained, which have become detached by the solution of the calcareous walls and septa of the shell, or are held together by the dove-tailed lobes of the margins of the chambers.

The Turritile is essentially an Ammonite disposed in spiral coils. The Hamites and Scaphites are other modifications of the outward form of similarly-constructed chambered shells: in the former the small extremity of the shell is curved, the rest being straight; in the latter both ends are curved towards each other like those of a canoe.

With none of these species has there ever been found a trace of the ink-bag; a part, indeed, of so delicate a texture that some surprise may be excited that any evidence of its existence could be met with in a fossil state. Not only the ink-bag, but the muscular mantle, fins, and cephalic arms, of extinct Cephalopods, with chambered shells, have been discovered in Oolitic strata, which have determined such fossil shells to belong to the second and higher order of Cephalopods.

Orden II.—DIBRANCHIATA, Owen.

Branchiae two, forming a pair, each with a branchial heart; funnel an entire tube; mantle muscular; an ink-bag; eight non-retractile acetabuliferous arms, with two long additional tentacles in most; eyes sessile; beak horny; shell internal (save in the females of one genus).

Compared with the Nautilus, the cephalic organs of prehension in all Dibranchiates are much reduced in number, the external ones, continued from the oral sheath, not exceeding eight, as in figs. 97, c, and 100, c, to which, in most of the genera, is added a pair of internal and much longer tentacula, as in figs. 97 and 100, d. The arms are much increased in size and of a more complicated structure, supporting on their internal surface numerous suckers, and sometimes connected together by a powerful muscular web. The eyes are much larger and more complex, are no longer pendunculated, but lodged in orbits (fig. 97, ee). The mouth (fig. 96) is armed with two piercing and trenchant horny jaws, e, resembling in shape and in their vertical movements those of the Nautilus. The gills (fig. 95, g) are two in number, each with a ventricle i expressly appropriated to the branchial circulation; the systemic circulation having a single muscular ventricle h, as in the Nautilus. The infundibulum b is a complete muscular tube, shaped like an inverted funnel. They possess a gland and membranous receptacle p for secreting and expelling an inky fluid. The sexual organs are in distinct individuals, as in the Tetranbranchiate order. All the species of both orders of Cephalopods are aquatic and marine.

The Dibranchiate order may be subdivided into two tribes; the one provided with the eight ordinary arms and the two longer tentacles, hence called Decapoda (fig. 97); the other tribe without the tentacles, and called Octopoda (fig. 101).

The various forms of the extinct Belemnitidae constituted one family in the Decapod tribe. The little Spirula, characterized by a less complex, but internal chambered shell, is the type of a second family. The cuttle-fish (Sepia, fig. 97), known by its internal calcareous shell (fig. 98), which feebly represents that of the Belemnite, exemplifies a third family of Decapods called Sepiidae. The common calamary (Loligo), in which the internal shell is reduced to a horny quill-shaped plate, represents the fourth and most extensive family of the present tribe, which I have called Teuthidae; and in which one genus (Enoploteuthis) had the caruncle of its acetabula produced into horny claws. In all the Decapods the mantle supports a pair of fins, and the siphon is generally provided with a valve.

In the tribe Octopoda fins are rarely developed from the mantle; but the eight ordinary arms are longer, thicker, and are united together by a broader web, which forms a powerful organ for swimming in a retrograde direction. One family in this tribe (Testacea) is represented by the genus Argonauta (fig. 102), in which, in the female sex, the first or dorsal pair of arms is dilated at its extremity into a broad thin membrane, like the mantle in the testaceous Mollusks; by means of these membranes the animal, in fact, forms for itself an extremely light, slightly flexible, and elastic, but calcareous, symmetrical shell, which is simple, and not divided into chambers; the vacated portion communicating with the rest, and being used by the inhabitant as the receptacle for the eggs (fig. 103). The siphon is without a valve, but is articulated at its base on each side to the inner surface of the mantle. The second family of the Octopods is termed Nuda, the species not being provided with an external shell (fig. 101). The first pair of arms is elongated, and contracts to a point; the funnel or siphon is without an internal valve or external joints. The rudimental shell is represented by two short styles, encysted in the substance of the mantle. The typical genus of this family is termed Octopus, in which the arms are provided with a double alternate series of sessile acetabula. In a second genus, Eledone, the arms are provided with a single series of acetabula. In the Sciadophorus a pair of filaments project between each of the suckers.

The skin of the naked Cephalopod is generally thin and lubricious, and can be more easily detached from the subjacent muscles than in the inferior Mollusks. In some of the smaller Cephalopods it is semitransparent; it is densest in the Calamaries, in which the epidermal system is most developed, as is exemplified in the horny rings or hooks upon the acetabula. In the Octopods the epidermis is reflected over the interior of the acetabula without being condensed into horn. Upon the body the epiderm may generally be detached in the form of a thick white elastic semitransparent layer. The second, or pigmental layer of the skin, analogous to the rete mucosum, consists of numerous cells of a flattened oval or circular form, containing coloured particles suspended in a fluid. The colour is rarely the same in all the cells; the most constant kind generally corresponds more or less closely with the tint of the inky secretion. In the Sepia there is a second series of vesicles containing a deep yellow or brownish pigment; in the Loligo vulgaris there are three kinds of coloured vesicles, yellow, rose-red, and brown; in the Octopus vulgaris there are four kinds of vesicles, red, yellow, blue, and black. In the skin of the Argonauta all the colours which have been observed in other Cephalopods are present, and contained in their appropriate cells. These cells possess the power of rapid alternate contractions and expansions, by which the pigment can be driven into the deeper parts of the corium, or brought into contact with the semitransparent epiderm. If the skin of an Octopus be slightly touched, the colour will be accumulated, gradually or rapidly, like a cloud or a blush upon the irritated surface. The Argonaut strongly exemplifies this chameleon-like power of change of colour.

The sole locomotive organs in the ordinary Octopods, and the sole prehensile organs in all the Dibranchiata are the appendages developed from the head, termed "arms," "feet," and "tentacles." They have no true homology with the locomotive members of the Vertebrata, but are analogous to them, inasmuch as they relate to the locomotive and prehensile faculties of the animal.

The eight arms of the Octopus commence by a hollow cone of muscular fibres attached by a truncated apex to the anterior part of the cephalic cartilage. The fibres are for the most part oblique, and interlace with one another in a close and compact manner, as the cone advances and expands to form the cavity containing the mandibulate mouth, at the anterior extremity of which they are continued forward, and separate into eight distinct portions which form the arms. The development of the eight external arms bears an inverse proportion to that of the body; they are longest in the short round-bodied Octopi, and shortest in the lengthened Calamaries and Cuttle-fishes, in which the two elongated retractile tentacles are superadded by way of compensation. These latter organs are not continued from the muscular cone which corresponds with the cephalic sheath in the Nautilus, but arise, like the internal labial processes in that Cephalopod, close together from the cephalic cartilage, internal to the origins of the ventral pair of arms. They proceed at first outwards to a large membranous cavity situated anterior to the eyes, and emerge between the third and fourth arms on either side.

The complex mechanism of the suckers of the arms is under the most complete control of the predatory Cephalopod. Mr Broderip states, that he has attempted, with a hand-net, to catch an Octopus that was floating within sight. with its long and flexible arms entwined round a fish which it was tearing to pieces with its sharp hawk's bill; the Cephalopod allowed the net to approach within a short distance of it, before it relinquished its prey, when in an instant it relaxed its thousand suckers, exploded its inky ammunition, and rapidly retreated under cover of the cloud which it had occasioned, by rapid and vigorous strokes of its circular web.

The Cephalopods which frequent the more open seas, and which have to contend with more agile and powerful fishes, have still more complicated organs of prehension. In the Calamary the base of the piston of the sucker is inclosed in a horny hoop with a dentated margin. In the Onychoteuthis the margin is produced into a long, curved, sharp-pointed claw. These formidable weapons are sometimes clustered at the expanded terminations of the tentacles (fig. 100, f), and in a few species are arranged in a double alternate series along the whole internal surface of the eight ordinary arms, as they were in the extinct Belemnite.

In connection with the uncinate acetabula at the extremities of the long tentacula of the hook-squids, may be observed a cluster of small simple unarmed suckers at the base of the expanded part. When these parts in each tentacle are applied to one another, they become locked together (fig. 100, e), and the united strength of both the peduncles d is thereby more effectually brought to bear upon any resisting object which may have been grappled by the terminal hooks. This is a very striking mechanical contrivance: human art has remotely imitated it in the fabrication of the obstetrical forceps, in which either blade can be used separately, or by the interlocking of a temporary joint be made to act in combination.

The brain (fig. 95, a) is inclosed in a cartilaginous cranium, together with a portion of the oesophagus, from which it is separated by the membrane analogous to the dura mater. Between that part of the fibrous membrane which lines the cerebral cavity and the pia mater covering the brain, there is an intervening space filled with a gelatinous arachnoid tissue. In the cuttle-fish, the supersophageal cerebral mass a consists principally of a cordiform body, superficially divided into two lateral lobes by a median longitudinal furrow. From the lower and lateral parts of this body proceed the short and broad optic nerves, which constitute the peduncles of the large reniform optic ganglions, and upon each peduncle there is placed a small spherical medullary tubercle. These tubercles exist also in the Calamaries, but appear not to be present in the Octopods.

From the inferior and anterior parts of the supersophageal mass, a thick cord descends on each side of the oesophagus, unites with its fellow, and dilates below that tube to form the anterior subesophageal ganglion, from which the nerves of the feet and tentacles arise. Two broader bands descend from the supersophageal mass behind the preceding, and form, by a like enlargement and union, the posterior oesophageal body, which blends laterally with the anterior one, and forms with it a large mass with a central perforation. Four short and slender chords, two of which are continued from the anterior spines of the optic lobes, and two from the anterior subesophageal lobes, converge forwards and unite to form a round flattened ganglion, which is closely applied to the back part of the fleshy mass of the mouth above the pharynx, from which are sent off the nerves to the different parts of the mouth. Two filaments from the pharyngeal ganglion descend to join a pair of ganglions below the mouth, homologous to the labial ganglions of the Nautilus. The nerves of the arm proceed from the anterior and inferior subesophageal ganglion, and correspond in number to the organs which they supply, being eight in the Octopoda, and ten in the Decapoda. The nerves answering to those of the shell muscles in the Nautilus form a single large pair, arising from the posterior angles of the subesophageal mass, and after a certain course outwards and backwards they expand into large stellate ganglions, from which the nerves of the mantle and of the pallial fins are derived. The branchial and visceral nerves and ganglions correspond pretty closely with those in the Nautilus.

With respect to the parts of the brain in the Vertebrata which are represented by the cephalic nervous masses in the Dibranchiate Cephalopods, we may regard the cordiform superior mass, which is principally in communication, and co-exists with the large and complex eyes, as the homologue of the optic lobes. The smaller supersophageal mass, anterior to the optic lobes in the Octopus and some other Cephalopods, may represent an olfactory lobe. The large subesophageal nervous mass, since it gives origin to the brachial nerves, to the acoustic and respiratory nerves, and to those two large moto-sensory columns which represent, by their structure, position, and distribution, the spinal chord of the Vertebrata, must be regarded as the representative of the medulla oblongata: it is obviously the part of the nervous centre which is most intimately connected with the vitality of the animal, and which is therefore here, as in the higher animals, the deepest seated and best protected part of the nervous system.

The Cephalopods are predatory and carnivorous animals. The curved, pointed, and trenchant mandibles (fig. 96, c, e), are encased upon a dense muscular cushion, e', and are protected by an outer lip a, and an inner lip b. The glottidium retains the true complex molluscan type (compare with fig. 29); but a greater proportion of it is uncovered by the tooth-strap, and forms soft caruncles suggestive of the possession of the faculty of taste. Their retractor muscle is shown at i. A fin is represented as in the duct of the salivary gland. In most of the Dibranchiata a second and larger pair of salivary glands is situated on each side of the oesophagus, at the commencement of the abdominal or hepatic cavity; their ducts unite to terminate below the tongue in the concavity of the lower mandible.

The peritoneal membrane is divided and disposed as in the Nautilus, in order to form special receptacles for the different viscera. The oesophagus is narrower than in the Nautilus, and provided with longitudinal plicae; it dilates soon after having passed through the cranium into a long ingluvies, forming a large cul-de-sac (fig. 95, e), at its commencement in both the Octopus and Argonauta; but in the Decapods it continues narrow and of uniform breadth to the stomach. This cavity (fig. 95, e) is an elongated sac, presenting, in the disposition of its muscular fibres, in the proximity of the cardiac and pyloric orifices, and in the thickness of the epithelial lining, the usual characters of the gizzard. The intestine, at a short distance from the pylorus, communicates with a glandular and laminated sac q, homologous with that in the Nautilus, and presenting a similar globular form in the Rossia and Loliopsis; but elongated and spirally convoluted in the Sepia and Loligo. It receives the biliary secretion between two broad lamellae, as in the Nautilus. The intestine is very short in all the Dibranchiata. In the Octopus it is bent upon itself (fig. 95, r), as in the Nautilus; but in the Sepia and Loligo it is continued forwards in a straight line, as at f (fig. 95), from the stomach to the vent. Its internal membrane is longitudinally folded, but is smooth at the short tract beyond the entry of the duct of the ink-bag; its termination is constricted either by the muscular fibres of the branchial septum, or by those which connect together the pillars of the funnel. In the Decapods provided with fins for swimming forwards, the anus can be closed by triangular fleshy valves; and in some species these are modified into the form of antennal filaments.

The liver is of large size in the Dibranchiata (fig. 95), but of more simple form than in the Nautilus. In the Sepia it is divided into two lateral lobes, which are notched at the upper extremity; in the Omychotethis it is a simple, elongated, compressed lobe, with undivided extremities; in the Octopus it forms a single oval mass, flattened anteriorly; in the Eleocone it is spherical, corresponding with the ventricose visceral sac. In the two latter genera the ink-bag (fig. 95, p) is enclosed within the capsule of the liver, and was naturally mistaken for the gall-bladder by some of the early anatomists of these Mollusca; but in the Argonaut, and in all the Decapoda, it manifests its distinct function by its separate position. The liver is surrounded by a smooth capsule, and is not subdivided externally into lobules, as in the Nautilus and lower Mollusca. The biliary ducts in the Octopoda are simple canals, which unite and terminate by a common orifice, in the pancreatic sac. In the Decapoda they receive the ducts of numerous clusters of caecal appendages beyond the smooth part of the liver.

The ink-bag consists of tough white fibrous texture, the outer surface of which is coated by a thin silvery or nacreous layer; its inner surface presents a fine spongy glandular texture. It presents a trilobate form in the Sepiola, and an oblong pyriform shape in the Sepia and Loligo. It is a very active organ, and its inky secretion can be reproduced with great activity. The tint of the secretion varies in different species, as is exemplified by the Italian pigment called "sepia," and the Chinese one called "Indian ink." It is of a very indestructible nature, as is exemplified by its frequent preservation in a fossil state in both the extinct Calamaries and the Belemnites. It is affirmed by some chemists to contain a peculiar animal principle, which Vizio has termed "melanine."

Many of the Cephalopods possess the power of emitting a luminous secretion. All of them are nocturnal and social animals, and are readily attracted by bright metallic substances.

The chief modifications of the circulating and respiratory systems of the higher order of Cephalopods are expressed in the characters of the order Dibranchiata.

The branchiae (fig. 95, g) are concealed, as in the Nautilus, by the mantle, which extends in front of the other viscera to form the branchial chamber, the infundibular muscular tube q projecting from its outlet. The rectum and the generative organs open into the branchial chamber at the base of the funnel, manifesting the same relation of the breathing organs to the termination of the alimentary canal which characterizes the lower orders of the Mollusca. Each gill consists, as in the Nautilus, of a number of triangular vascular laminae, extending transversely from either side of the fleshy stem, and decreasing in size to the extremity of the gill. Each plate is composed of smaller transverse laminae, which are themselves similarly subdivided; the entire gill presenting the tripinnate structure, which affords the most extensive surface for the minute subdivision of the blood-vessels. In the Loliopsis each gill has twenty-four pairs of plates; in the Sepia, thirty-six pairs; in the Loligo sagittata, sixty pairs. The stem of the gill is not only attached by its base, but by a thin fibrous membrane through nearly its whole length to the mantle.

The mechanical part of the respiratory act is performed by the muscular actions of the mantle and funnel, the gills not being provided with vibratile cilia, as in many of the inferior Mollusca. The water is admitted into the branchial cavity at the anterior aperture of the mantle, outside the base of the funnel. Two large valvular folds of fibrous membrane, which are concave towards the respiratory cavity, prevent the currents from escaping by this entry. They are therefore propelled by the whole force of the contraction of the muscular mantle through the cavity of the funnel, the base of which is articulated, in most of the Cephalopods, by lateral joints, with the sides of the anterior aperture of the mantle.

In the male Cephalopod, the testis, lodged at the bottom of the visceral sac, consists of a membranous pouch, to one part of the inner surface of which are attached numerous dichotomising blind spermatic tubuli. These swell and burst at the breeding season. The spermatozoa are conveyed along a convoluted sperm-duct, where they are moulded, with added mucus, into a slender cylindrical coherent thread; this next passes into a wider tube, with glandular walls, where the thread divides into packets of spermatozoa, which become inclosed in albuminous sheaths; a glandular sac or caecal tube (called "prostatic") communicates with the vesicular canal; and this finally opens into the pouch in which filamentary packets of sperm-material, having acquired the requisite mechanism, exhibit the movements which first attracted the attention of Needham.

These moving filaments in the terminal pouch, or capsules of spermatozoa and sperm-fluid, with a peculiar associated mechanism, of which an internal spiral spring is the most conspicuous part, form one of the most remarkable peculiarities of the Cephalopoda, and have been regarded as parasitic worms, under the names of *Echinorhynchus*, *Scolex dibothrius*, *Neelthamia expulsatoria*, &c., by different comparative physiologists. They are now denominated "spermatophora;" and they parallel in the male the albuminous packets of ova in the female. The efficient cause of their movements appears to be a combination of the contractility of the external sheath and sperm-receptacle, with the elasticity of the internal spiral membrane, and the phenomena of endosmosis. The final intention of the super-addition of protecting sheaths for the semen, like those for the ova, appears to relate to the safe conveyance of the spermatozoa to the ova of the female, there being apparently no true intromission in the Cephalopoda. The peculiar mechanism of the sperm-receptacles insures their rupture and the dispersion of their contents after their brief transit through the sea-water.

The female organs consist in the Dibranchiate Cephalopods, as in the *Nautilus*, of ovarium, oviduct, and superadded nidamental glands, but with several modifications in the efferent part of the apparatus. The ovary (fig. 95, a) is always single, and the ovisacs, characterized by their elliptical form and reticulate parietes b, are attached to one part of its cavity, as in the *Nautilus*. In the cuttle-fish there is a single oviduct, with a glandular laminated outlet; and there are two distinct laminated nidamental glands on each side of its termination. In the *Octopus* there are two oviducts, which in the *Octopus* and *Eledone* are each provided with a special glandular enlargement about the middle of their course; but there are no detached nidamental glands. In the *Loligo* there are two distinct convoluted oviducts, and two separate nidamental glands. These glands in the cuttle-fish rest upon a soft parenchymatous body, of a bright orange colour. The corresponding part is rose-coloured in the *Sepiola*; it is double in the *Rossia* and the Calamaries. These bodies have no ducts.

**Tribe I.—DECAPODA.**

Arms eight, tentacles two.

**Family I.—SPIRULIDÆ**, Woodward.

Shell internal, nacreous, discoidal, whirls separate, many-chambered; the involute spire next the ventral surface of the body; septa concave, next the outlet; siphuncle near the concavity of the shell-curve; mantle with a tricuspid front-border, bilobed behind, and terminated by a sub-circular fleshy disc, with a narrow lamelliform appendage on each side; at the notches on the dorsal and ventral sides, above the disc, are exposed parts of the last whirl of the shell.

**Genus Spirula**, Lam.—Body oblong; arms short, with six rows of minute suckers; tentacles long, cylindrical, funnel-valved, and with elongated basal lateral joints with the mantle. The internal organization of the *Spirula* accords with the Dibranchiate type, and exhibits the modifications characterizing the Decapodous tribe, such, e.g., as the glandular follicles upon the hepatic ducts, the absence of the septum or framen of the branchial chamber, the appendages to the branchial hearts, the absence of an ingluvial dilatation of the gullet, the division of the liver into two lobes, and the separation of the ink-bag from the liver.

The most perfect known example of the *Spirula* is in the collection of Mr Cuming. It was picked up by Mr Percy Earl on the coast of New Zealand. This specimen is nearly three inches in length from the pallial disc to the tips of the arms; only the clavate ends of the tentacles seem to be wanting. It is figured in the *Annals and Magazine of Natural History*, vol. xv., pl. 15, and in L. Reeves' *Elements of Conchology*, part i., plate A., figs. a, b, c. The names *Spirula levis* and *Spirula australis* have been proposed for this specimen. The species *Spirula reticulata* (Owen, in the *Zoology of the Samarang*, *Mollusca*, pp. 14-16, pl. iv., figs. 3 and 9) is founded on a mutilated specimen, taken by Mr George Bennett, F.L.S., on the coast of Timor.

If the *Spirula* taken in the Atlantic Ocean, and figured by Péron in the *Atlas du Voyage aux Terres Australes*, tab. xxx., fig. 4, be there accurately delineated, it may be regarded as a distinct species, and retain the name of *Spirula Péronii*.

The *Spirula* is widely diffused over the warmer parts of the ocean, especially in the southern hemisphere. The delicate shell is strewed in abundance over parts of the coast of New Zealand, and a few specimens have been brought by the Gulf Stream to the south-west coast of England.

**Family II.—BELEMNITIDÆ**, Owen, Gray.

Shell internal; the chambered and siphunculated part, called "phragmocone," is straight, conical, and is lodged in a sheath or guard, produced more or less anteriorly into a calcareo-albuminous plate, and behind into a solid "mucro," varying in length and shape. This is the part commonly known as the "belemnite." The septa of the phragmocone are concave next the outlet or base, and the siphuncle is at the ventral side. All the genera and species of the family are extinct. The acetabula of the arms were provided with horny hooks. They range from the lias to the gault.

**Family III.—SEPIADÆ**, Woodward.

(*Cuttle or Cuttle-fish.*)

Trunk or pallial part (fig. 97, a, b) sack-shaped, bordered on each side with a narrow longitudinal musculo-dermal fin bb; shell (fig. 98) internal, gelatinocalcareous, friable, consisting of a flattened closely-laminated phragmocone bc, lodged in an open expanded guard aa, prolonged behind into a "mucro."

**Genus Sepia**, Linn. (as restricted by Cuvier).—The septa of the phragmocone are very numerous and close-set; they are united by finely-undulated vertical lamellae (fig. 98, A). The whole shell is light and porous. It is still used as "pounce," and for polishing the ivory plates of miniature-painters. It is sometimes given medicinally as an antacid, the hardening salt being pure carbonate of lime. Arms (fig. 97, c, e) with four rows of suckers; funnel valved, and articulated to two lateral tubercles on the mantle.

Sp. *Sepia officinalis*, Linn. (fig. 97).—a, constriction between "soma" and "pallium"; b, pallial fins; c, cephalic or somatal arms; d, clavate tentacles; e, eyes. The left ventral arm in the male has many of the suckers on its basal half very small or oblate; and the arm is there expanded, with the inter-acecular membrane pitted and reticulate. The spermatophora are attached in coitus to the lip of the female. (Steenstrup.)

The eggs of the Cuttle-fish are of comparatively large

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1 An account of some microscopic discoveries in the Calamary, and its wonderful milk-vessels, Svo, 1745. 2 Description of certain Belemnites, preserved with a great proportion of their soft parts, &c., in the Philosophical Transactions, 1844. Some of the species here described have been formed into a sub-genus, called Belemnotus; but they possess the "phragmocone" and "guard," which are the essential characters of the extinct family. 3 Annals and Magazine of Natural History, August 1837. enveloped in its proper horny covering, which is prolonged into a pedicle at one extremity, and attached thereby to some foreign body. Many of these egg-capsules are generally found clustered together. They are called "sea-grapes" by the fishermen.

**Family IV.—Teuthidae, Ow.**

*(Calamaries, Squids)*

Shell internal, reduced to an aluminous or horny plate, consisting of a stem or shaft, and two lateral expansions or wings. It is called the "gladius" or pen (fig. 99).

**Sub-Family I.—Oegopsidae.**

Eyes exposed; fins terminal and united.

**Genus Ommastrephes, D'Orb.**

*(Flying-Squids).*—Gladius with three diverging ribs, and a hollow conical appendage; body long and cylindroid; fins terminal, large and rhombic; arms with two rows of suckers; tentacles with four terminal rows of suckers; eyes not covered by the integument. Muscular chord on the funnel.

Sp. *Ommastrephes sagittatus*

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(Loligo sagittata, Lam.)—In the sagittated Calamary the male has a shorter body and longer and stronger arms, the second and third pairs of which have relatively larger acetabula than in the female. The spermatophora are attached in coitus within the mantle, to its wall or to part of the viscera of the female. The species of this genus are gregarious, and frequent the open ocean. With the best organs of vision, they manifest the highest powers of locomotion in the Molluscous province, leaping out of the water so high as sometimes to fall on the decks of ships; whence they are called "flying squids." They form the chief food of the cetaceous dolphins and cachalots, and of the albatross and larger petrels. They are used as bait in the Newfoundland fisheries.

**Genus Onychoteuthis (Hook-Squids).**

Gladius lanceolate, with a hollow conical stem-base; arms (fig. 100, c, e) with two rows of suckers; tentacles d long, with the clavate ends provided with a double series of hooks f, and usually with a group of suckers at the base, which, uniting with that of the opposite tentacle, as at e, fig. 100, enables both to act in conjunction; terminal fins unite to form a rhomb.

**Genus Enoploleuthis, D'Orb.**

Gladius and terminal fins as above; arms provided with a double series of hook-bearing pedunculate acetabula; tentacles long, with terminal hooks. The hook-squids are solitary, frequenting the open sea, often near the banks of sargasso-weed.

A specimen of the second genus (*Enoploleuthis ungulata*) was taken during Cook's first voyage, in the South Pacific, which must have been six feet long from the tips of the arms to the end of the mantle. The huge rhomboid pair of fins are still preserved, dried, in the Hunterian Museum of the London College of Surgeons, together with spirit-preparations of sections of the uncinated arms, the heart, and the mouth. Fig. 96 is taken from the last specimen, but is reduced to one-half the natural size.

The natives of the Polynesian Islands, who dive for shell-fish, have a well-founded dread of these formidable Cephalopods.

**Genus Loligopsis, Lam.**

Arms with numerous small acetabula; the third pair usu-

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1 Gr., signifying "eyes which turn." ally the longest, the first the shortest; tentacles very long and slender, rarely preserved in captured specimens; body conical, tapering to the extremity, at or near which is a pair of rounded fins; no infrabuccal valve; mantle sub-transparent, widely open anteriorly.

Sp. *Loligopsis Zygema*.—This species has the tentacles acetabulate throughout their length. The *Loligopsis Veranyi* is remarkable for the great length of the tentacles; the suckers are limited to the clavate ends. The *Loligopsis vermicularis* has the slender head longer than the longest arm.

**Genus Leachia**, Stp.—The *Leachia Reinhardii* is distinguished by its strong armature of cartilaginous bands on the mantle; one, denticulate, along the middle of the back; arms in length, 3, 2, 4, 1, or each with two series of suckers.

Sp. *Leachia hyperborea*, Stp.—Fins narrow, extending along half the length of the body, and forming together a lanceolate figure; arms, as to length, 3, 2, 1, 4, with very large acetabula; tentacles twice as long as the arms.

**Sub-family 2.—Myopsidae.**

Eyes covered (wholly or almost) by skin.

**Genus Cranchia**, Leach.—Body large, ventricose; fine small, terminal; mantle supported in front by a branchial septum; head very small; arms short, with two rows of suckers; tentacles with four rows of suckers on the clavate ends; funnel with a valve; gladius long and narrow.

The *Cranchia scabra*, Leach, a species about two inches in length, was taken off the west coast of Africa, during Tuckey's Congo expedition; the genus founded thereon was named in honour of Mr Cranch, naturalist to the expedition.

**Genus Loligo**, D'Orb. (part of the genus *Loligo* of Lamarck), (*Calamaries* or *Common Squids*).—Body elongate, especially in the male, tapering behind, with a pair of rhomboidal, terminal, more or less extended, fins; gladius narrow, pen-shaped, with the shaft produced in front—it is multiplied by age, several being found packed closely, one behind the other, in old specimens; arms with two rows of pedunculate suckers; tentacular club with four rows of suckers, the fourth left arm in the male, with the suckers of the terminal part, gradually disappearing, whilst their peduncles increase, and assume the form of long papillae, especially along the outer margin, giving the extremity of the arm a pectinate appearance; spermatophora attached, *in coitu*, to the fringed lip of the female.

In *Loligo vulgaris* the suckers of the peduncular club are very large on the two central rows, and very small in the lateral rows; the horny ring of the central suckers is finely toothed along half its circumference.

In *Loligo Forbesii* the central rows of the peduncular suckers are scarcely larger than the lateral ones, and not more than one-third larger than the largest suckers of the third arm; the horny ring of the peduncular suckers is toothed all round. A specimen of the species has been taken which was 2 feet long, including the tentacles.

These squids are good swimmers; they also crawl, head downwards, on their cephalic disc. The ova are inclosed in long gelatinous cylindrical sheaths, and offer a close analogy to the spermatophora in the male. Bohadsch estimated a cluster of these capsules to contain nearly 40,000 eggs.

**Genus Sepioteuthis**, Bl. (*Long-finned Squids*).—Gladus lanceolate, narrowed anteriorly, convex dorsad; lateral pallial fins as long as the mantle.

In *Sepioteuthis sepioidea* the fourth left arm in the male has the outer row of suckers of the terminal part metamorphosed into compressed leaf-like papillae, united by a membranous bridge with the roots of the peduncles in the opposite row, which are transformed into blunt elevations. The fourth right arm has the terminal suckers scarcely discernible, and probably assisting the left during the generative act.

The spermatophora are attached, *in coitu*, to the fringed lip of the female (Steenstrup). The species are distributed in the Mediterranean, Indian, South Atlantic, and Australian Seas.

**Genus Loliolus**, Stp.—Internal shell horny, broad, with the shaft sharp-keeled; no muscular chords to the funnel, and suckers without the elevated band round the horny ring, thus resembling *Sepiola* and *Rossia*.

In *Loliolus typus* the left fourth arm in the male is without a trace of sucker; the acetabular bed being converted into a compressed obtusely-dentate edge, produced by the confluent bases of the peduncles of the inner series of suckers.

**Genus Sepiola**, Leach.—Abdomen short, purse-like; fins dorsal, rounded, contracted at the base; no muscular bands to the funnel; no elevated band round the horny ring of the suckers; mantle continuous with the back of the head.

In the *Sepiola Rondetii* the left dorsal arm in the male is expanded by the confluence of the elongated stalks of the suckers, especially of the inner row, and by a production of integument at the base of the inner side of the arm, (Stp.)

**Genus Rossia**, Ow. (so named in honour of Sir James Clark Ross, R.N., the arctic and antarctic navigator).—Body short, ventricose, with subdorsal rounded fins, no muscular bands to funnel; posterior border of the mantle free; the dorsal pair of arms in the male obliquely twisted inwards, with about eleven of the outer row of suckers elevated on long peduncles, the roots of which are compressed, leaf-shaped, and surrounded by dermal folds or processes.

"In one of my males I found two soft envelopes of spermatophora between the cutaneous folds of the arm." (Stp.)

Sp. *Rossia palpebroa*, Ow.—This is the largest known species of the genus. It is characterized by numerous small suckers on the tentacles. A fold of integument is reflected, like a large lower eyelid in front of the eyeball. The species was obtained by Captain Sir James Ross at Elwin Bay, Prince Regent's Inlet. The utility of the peculiar defence of the eyes, in a species living in seas sometimes charged with spicule of ice, is obvious.

Sp. *Rossia Melleri*, Stp.—This is distinguished by very large suckers on the tentacles, in both males and females; the middle row exceeding in size the largest brachial suckers. Hab.—Greenland.

The *Rossia dispar*, Rupp. (*Heteroteuthis*, Gray), has very large suckers on the third pair of arms.

The *Rossia Ocenini*, Ball, agrees with the male, and *Rossia Jacobii*, Ball, with the female, of the *Rossia macroscoma*, of D'Orbigny, and of Forbes and Hanley.

**Genus Histiotethus**, D'Orb.—Trunk short, bursiform, with a pair of terminal subdorsal rounded fins; palpebral orifice large; first, second, and third pair of arms united by a web to within one-fourth their length from their tips.

Sp. *Histiotethis bonelliana*, D'Orb.—This species was discovered by M. Verany in the Gulf of Genoa. The remarkable form of Cephalopod of which it is the type is beautifully figured in plates 19, 20, and 21 of the *Mollusques Mediterranens* of that excellent Malacologist.

**Section Octopoda**, Leach.

Arms with sessile suckers, no tentacles; trunk united to the head by a broad nuchal band; branchial chamber divided by a longitudinal partition; two oviducts, without nidamental glands.

**Family 1.—Pinnata**, Owen.

A pair of advanced subdorsal fins from the mantle.

**Genus Sciadephorus** (*Cirroteuthis*, Eschricht).—Palial fins rounded; arms united by a web nearly to their tips; suckers in a single row, alternating with cirri. In the male a portion of the suckers seems, as it were, stripped off the lower third of the right arm.

Sp. *Sciadephorus Mulleri* (*Cirroteuthis Melleri*, Eschricht).—Hab. coast of Greenland. No pallial fins. Shell represented by two short styles in the substance.

Genus Eledone, Leach.—Arms with a single series of suckers. In the male the third right arm is shorter and somewhat thicker than the left one, with fewer acetabula (sixty-four instead of ninety-three in Eledone moschata). A strong cutaneous border commences in the middle, on the margin of the membrane, stretched between the third and fourth arms, and thence runs along the arm to its apex, where the terminal plate is furnished with several longitudinal folds. All the other arms have membranous laminae instead of suckers at their extremity. (Stp.)

The Mediterranean species, Eledone moschata, is so called on account of the musky odour it emits.

Genus Octopus, Cuv. (Poulps).—Arms with a double alternate series of suckers united at their base by a web. The species of Eledone and Octopus were the "polypi" of Aristotle. They swim by vigorous contractions of the interbrachial muscular web, darting with the rounded end of the trunk forwards. They creep on shore with the same part upwards, and with their eight arms, like the legs of a spider, sprawling over the surface, in a rotating shuffling manner. The ink which they discharge is of a dark chestnut-brown colour. In the male the third right arm is much shorter, but is as thick or thicker than the left, with fewer acetabula, and bearing externally at its apex a longish plate provided with a greater or less number of transverse ridges with intervening pits. A muscular fold of skin connects this plate with the web at the base of the arm, the fold running down the dorsal margin of the arm; which margin is so rolled towards the inner side of the arm as to form a more or less closed channel, probably to conduct the spermatophora to the apical plate. The female oviposits on sea-weeds or in empty shells.

"Octopi of enormous size are occasionally met with among the islands of the Meia-co-shimah group. I measured one, which two men were bearing on their shoulders across a pole, and found each brachium rather more than 2 feet long, giving the creature the power of exploring an area of about 12 feet without moving, taking the mouth for a central point, and the extremities of the arms, to describe the circumference." (Adams.)

Genus Tremoctopus.—Dorsal pairs of arms (in the female) webbed to or near to their ends; suckers in two rows; conspicuous aqueous pores on the back of the head.

In the male of Tremoctopus carens (fig. 101) the arms are not webbed; but the third right arm df is enlarged with a double row of numerous (forty in each row) suckers, and a terminal disc f; a cavity in this arm contains a long convoluted spermatophore. It is deciduous in coitu, and adheres to the female, usually to the inner side of the mantle, where it was mistaken for a parasitic worm (Hectocotylus, Cuvier) when first discovered. This hectocotylised arm is further distinguished from the rest by being white or colourless.

Family III.—TESTACEA, Owen.

Dorsal arms of the female (fig. 102, 1), with much expanded terminal membranes, serving to secrete and sustain a symmetrical involuted nidamental shell. The third left arm in the male is "hectocotylised" and deciduous in coitu.

Genus Argonauta, Linn.—The characters of the family are those of its sole representative genus.

Sp. Argonauta argo, Linn.—On this species, which inhabits the Mediterranean, and is found in tolerable abundance at Messina and Sicily, Madame Power made her observations and experiments, which finally determined the

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1 Catalogue of Mollusca in British Museum, Part I., Cephalopoda Anteapodæ, pp. 5 and 14. 2 Memoire sur un ver parasite d'un nouveau genre (Hectocotylus), Annales des Sciences, vol. xviii., 1829. 3 Aristotle, describing this species of "Polypus," wrote, "differt mas e femina eo, quod habet corpus oblongius, et genitalia, quod e piscatoribus vocatur, in brachio, album." (Hist. Anim., lib. v., c. 10, l. ed. Schneider, p. 196.) 4 The following is Prof. Steenstrup's summary of the sexual characters presented by the males of the Cephalopoda:

| OCTOPODA | Argonauta | Third (left) arm a Hectocotylus, deciduous, colourless, developed in a sac. (Feminæ polyandriae.) | |----------|-----------|---------------------------------------------------------------| | | Tremoctopus | Third right arm hectocotylised, permanently attached, coloured, developed in a free state. (Feminæ monandriae.) | | | Octopus | | | | Eledone | |

| DECAPODA | Rossia | First left arm hectocotylised with the right one only in the middle. | |----------|------------|---------------------------------------------------------------------| | | Sepiidae | alone, in its whole length. | | | Sepia | Fourth left arm hectocotylised at the base. | | | Sepiatenthis | Fourth left arm hectocotylised at the apex. | | | Loligo | Fourth left arm hectocotylised at the apex. | | | Lolilus | Fourth left arm hectocotylised at the apex. | | | Ommastrophes | Fourth left arm hectocotylised at the apex. | | | Oxychotenthis | Fourth left arm hectocotylised at the apex. | | | Loligopsis | Fourth left arm hectocotylised at the apex. |

no hectocotylised arm yet observed. The question arose out of the anomalous relations of the shell to its occupant; the shell not being attached by any muscle to the Cephalopod, and this having been observed to quit the shell and survive in captivity for some time without attempting to resume it. Madame Power first observed and published the function of the brachial membranes in maintaining the shell in proper relation to the body, as shown in fig. 102, 1, where the membrane is partly retracted from the shell, which it can wholly cover. She likewise demonstrated, experimentally, the function of those membranes in the formation and separation of the shell. She removed portions of the shell from healthy argonauts imprisoned in her aquaria, and preserved them long enough to witness the mode and degree of reproduction of the mutilated parts of the shell. She cut off one of the membranous arms, and preserved the mutilated argonaut long enough to show that the growth of the side of the shell next the amputated arm did not proceed; whilst that which remained covered by the membrane of the entire dorsal arm had received a notable increase. Specimens of the shells, showing that inequality of the two sides of the shell as the result of the experiment of removing one membranous arm, were transmitted by Madame Power to the author of the present article, by whom the experiment was suggested to that lady.

The argonaut shell is, however, peculiar to the female sex. It has the special function of an incubating and protective nest. It is not the homologue of the camerate shell of other testaceous Cephalopods, nor of the internal rudimental shell in naked Cephalopods. It is to be regarded as answering to the cocoon which is secreted for a similar office by the leech and in many Articulata, and to the nidamental float in Jambuina.

Fig. 103 shows the place of attachment of the eggs, which are appended by filamentary stalks to the involuted spire of the shell. They are usually compacted in that part of the shell, and concealed by the body of the parent argonaut. This figure, and fig. 102, are of the species described by Mr Adams as the Argonauta gondola, of which he writes:

"There is a considerable difference in the general aspect and disposition of the spots, &c., between the animals of Argonautae gondola and argo. In A. gondola the sac-like mantle is more ovoid and elongated; the head is narrower; the funnel broader, shorter, and furnished, at the upper and anterior extremity, with two conical elongations; the eyes are considerably larger and slightly more prominent; the tentacular arms are much shorter in comparison and of greater width, more particularly at their basal portions; the suckers are much larger, more prominent, and placed closer together. This species varies also considerably in colour from A. argo. The extremities of the brachia are marbled with deep-red brown, and, in the other parts, are covered with large, irregular, oval, reddish blotches, each margined with a dark colour; the circumference of the suckers is marked with brown spots; the upper surface of the funnel is covered with pale pink, rather scattered and irregular, quadrate blotches, margined with dark-red brown; the mantle, on the dorsal surface, is densely sprinkled with round and square spots of a chestnut-brown and crimson of different sizes; the velamenta are minutely dotted with crimson and red-brown, and have a more bluish tinge than those of A. argo; the under surface is mottled and minutely dotted with dark chocolate on the arms, and on the body is marked with small, irregular, dark red-brown spots.

In the specimen of A. gondola from which the accompanying drawing was made, the ovary was distended with ova, but in a much less advanced stage of development than those deposited in the shelly nidus. Some of these latter were sufficiently matured to enable me to trace, under the microscope, the early indications of the being of the argonaut; and although the progress is not followed very far, it is sufficient to ascertain the similarity with the changes observed by Poli in the same genus, with whose writings I afterwards compared my remarks; the only difference of any importance appears to be that Poli regarded as the shell what I have called the yolk-bag. At first, the ova are semi-opaque, pale yellow, and apparently speckled minutely, which is owing to the granular yolk; afterwards they become clouded with light brown blotches, and three dark spots make their appearance, one for each eye and one for the viscera; these spots, in the next stage, approach each other, and a faint outline of the future argonaut is visible, a club-shaped embryo, rounded in front and tapering behind. The front part is then lobed; a black mark for the horny mandibles is perceived, and the eyes are large and prominent; the yolk-bag, or vitellus, is next seen very distinctly, and the processes extending from the head are more elongated." (Loc. cit., p.7.)

The most complete series of observations on Cephalopodous development are those recorded by Kölliker, of the cuttle-fish (Sepia officinalis). The primary germ-cell... divides and subdivides, but assimilates only a small proportion of the vitellus to form the germ-mass; the rest of the yolk being taken in for food after development of the embryo, as in the oviparous shark and bird. What is called a "germinal area" is thus formed upon the yolk. The basis of the trunk or mantle seems to be the first to appear; but almost simultaneously follow parts which are afterwards recognised as the "funnel-lobes," "gills," and "eyes." Next appear parts of the head called "anterior" and "posterior" cephalic lobes; then the arms, beginning by the ventral pair, and all these parts, save the gill-rudiments, manifest ciliary action. The lobes or crura of the funnel elongate, the eyes become reniform, the mouth is established as a median semilunar depression with a raised border. The mantle first liberates a border at its ventral side (some Cephalopods, e.g., Sepia octopus, retain this stage); afterwards the free border extends all round, and by its growth begins to cover the gills. The crura of the funnel approximate, the cephalic lobes coalesce, the bases of the dorsal pair of arms extend, approximate, and complete the encompassing of the mouth. There is nothing in the developmental relations of the cephalic arms that countenances the idea of their homology with the repent ventral disc of Gastropods. The crura of the funnel first coalesce at their dorsal margin, and in Nautilus the development of the tube is arrested at this stage; but the ventral overlapping borders of the funnel-lobes next coalesce, completing the tube. The alimentary canal, with the liver and ink-bag, are successively developed. The molluscous bend of the intestine is formed as the mantle-cavity rises from the germinal area; the anus having been previously established between the gill-rudiments and the ventral ends of the funnel-lobes. The vitelline sac is drawn in between the mouth and vent, and becomes divided into an internal and external yolk; the latter becoming progressively "internal" until finally absorbed. At a certain stage of pallial growth alternate contractions and expansions of the mantle indicate the respiratory movements. The ink-bag is conspicuous by the colour of its contents, which suffice to blacken a considerable quantity of water. At the period of exclusion five layers of the shell of the young cuttle-fish have been formed; but, except the nucleus, which is calcified, they are flexible and transparent; the internal shell of the Teuthidae is arrested at this stage. The pallial fins are relatively broader than in the mature animal, and the cephalic arms are furnished with both suckers and a basal web. Thus the little cuttle is enabled to swim either backwards or forwards, and its eyes have acquired the requisite development to direct it to its appropriate food, or warn it of an approaching enemy, from which it has also the means of concealing itself by its already developed ink-bag.

In the class at the summit of the Molluscous series, as in the Arachnida of the Articulate series, there is no metamorphosis. The Cephalopod character is manifested before the parts of the embryo are completed; even the Dibranchiate peculiarity of the infundibular cartilages is recognisable, when as yet, only the visceral sac, funnel, gills, and eyes have been outlined on the germ-mass. No phase or form of Molluscous existence below the Cephalopod is transitorily manifested. Before the ciliary action is visible on the germ-mass, the parts that afterwards exhibit it bear the Cephalopodous stamp. The retained conditions which are transitional in the embryo Sepia are peculiar, as has been pointed out, to certain other Cephalopods. Were growth superinduced at any arrested stage of Cephalopodous development, no known inferior form of Mollusk would result; and no arrested stage of Vertebrate development would produce anything like a cuttle-fish.

The notices of the habitats of the species selected to illustrate the several divisions of the Molluscous province, will have served to convey a general idea of their distribution in space. It remains only to sum up briefly the relations of the Mollusca to time.

In the oldest known fossiliferous deposits—Cambrian or Lower Silurian—this province of life is represented by species of Brachiopoda (Orthiina, Pseudocrania), of Lamellibranchiata (Lyrodema, Modiopsis), of Gasteropoda, both Nuculeibranchiate (Moclurea, Belerophon) and Scutibranchiate (Rhaphistoma, Holopaza), and of Tetrabranchiate Cephalopoda (Gonoceras, Lituites, Endoceras). Thus the leading modifications or classes of the Mollusca appear simultaneously at the earliest beginnings of life to which our present knowledge reaches. Some additional but allied species of the same families appear in the Upper Silurian beds. In the Devonian system of rocks the Brachiopoda are principally represented by Spiriferida, and Producta here makes its first appearance; the Lamellibranchiata are exemplified by Megalodon and Pterinea; the Cephalopoda by Clymenia and Bacrites. The Goniatites, which make their first appearance in the Devonian, flourish most in the Carboniferous series. New Brachiopoda—e.g., Camarophoria—first appear in the Permian, at which period the Lamellibranchiate Myalina was most abundant. A Permian Bivalve has been referred to the genus Lima; the Brachiopodous Lingula and Crania have continued to be represented from the Cambrian epoch to the present day; but most of the Molluscous genera of the Palaeozoic rocks are extinct.

True Ammonites first appear in the Trias, and the Tetrabranchiate Cephalopods were most abundant and various at the Oolitic periods, when the Dibranchiates, under the form of Belemnites, first appeared on the stage of life. In the same secondary strata the Gasteropoda are chiefly represented by asiphonate genera. The siphonated Aporrhais and Pyura first appear in the Cretaceous strata. Fresh-water Pulmonifera occur in the Purbeck beds; but terrestrial species have not been found in strata older than the Tertiary. The Lamellibranchiate genera are reckoned by Woodward to be seven times more numerous in the newer Tertiary than in the Palaeozoic strata. In the latter the genera belong to the families with an open mantle. "The siphonated Bivalves do not appear till the middle of the secondary age, and are only now at their maximum." (Manual, p. 418.) Fossil shells have afforded the readiest mode of testing and characterizing the chief divisions of the Tertiary system. To those strata in which a small per-centage is only referable to recent species the term "Eocene" is given, as if the dawn of actual life had then appeared. Strata of the "Miocene" age are those that have not more than 50 per cent. of recent species. When they constitute from 50 to 70 or 80 per cent. they characterize the "Pliocene" Tertiary beds.

The natural families of the Mollusca which seem now to be verging towards extinction are the Rhynchonellida, the Trigoniidae, and the Nautilidae. The following seven families have altogether passed away:—Productidae, Orthidae, Spiriferidae, Hippuritidae, Orthoceratidae, Ammonoidea, and Belemnitidae.

In this retrospect of Molluscous organization, so far as it can be carried through the dark vistas of geological time, we discern "an ascent and progress in the main." Lamellibranchiate have superseded Palliobranchiate Bivalves; Siphonate have succeeded Asiphonate Univalves; and the Dibranchiate now vastly outnumber the Tetrabranchiate Cephalopods.

(R.O.)

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1 Entwickelungs geschichte der Cephalopoden, 4to, 1844. 2 Ibid., taf. ii., fig. xvi.