The subjects of the animal kingdom to which, in this article, we propose to direct the attention of our readers, were considered as related to Insects by the authors of the most popular systems of zoology of recent times. In common life they are confounded with animals of a very different character, under the general denomination of Shell-fish, and are familiarly known by particular names, as crabs, lobsters, shrimps, and prawns.
The ancients, in their contemplation of shell-fish, denominated such as were furnished with true shells, Ostreæ, or testacea, whilst such as were covered with a softer crust were termed Mollusca, or crustacea. The former group comprehends those molluscous animals which are usually described in our systems of conchology (see the article MOLLUSCA of this work), whilst the latter will form the subject of the following remarks.
The crustacea belong to that extensive division of Anneloid animals which have been denominated Condylopoidea, on account of the regularly jointed structure of their limbs, and which also includes spiders, insects, and millipedes. They differ, however, in many respects from the three last groups, especially in the greater perfection of their circulating system, and by the adaptation of their respiratory organs to water or damp air. The true character of the differences will be rendered more obvious while we unfold the physiology of the crustaces, preparatory to a few remarks on their systematical arrangement.
CHAPTER I.
PHYSIOLOGY OF THE CRUSTACEA.
Crustaceous animals exhibit various modifications of head, body, and tail. The head bears the antennæ, the eyes, and the mouth, while the body and tail support the organs of locomotion and respiration. The body, above, is usually protected by a thorax or shield, marked with grooves or eminences corresponding to the position internally of the more important organs. These parts have been denominated regions, with annexed terms indicative of the organs which they cover. Thus the stomachal region is situate in the middle of the shield, and immediately behind the eyes, and is followed towards the centre of the shield by the genital region, to which succeeds the cardial region. Between the cardial region and the junction of the shield with the tail is the posterior hepatic region, the anterior hepatic region being situate on each side of the stomachal one. The branchial regions extend on each side from the genital and cardial regions to the margins of the shield. These distinctions, which were suggested by the craniological speculations of Gall, are of some value in the discrimination of nearly allied species, and have been successfully employed by the industrious Desmarest in illustrating the character of such as occur in a fossil state, and which in general have their members mutilated or bruised.
SECT. I.—EXTERNAL COVERING.
The covering of crustaceous animals may be considered as composed of a cuticle, a crust or scale containing much earthy matter, and a thin membrane representing the true skin of the vertebral animals. The cuticle in some cases is extremely thin, while in others it is developed into prominent hairs and other similar appendages. The crust contains, besides the animal matter which forms its basis, and which in the lobster constitutes about a fourth of the whole, the carbonates and phosphates of lime, the former occurring to the amount of sixty per cent., the latter to fourteen per cent. in the animal referred to. The flexible laminated matter of the crust remaining and retaining its form after the earthy salts have been dissolved by acidulated water, has been shown by M. Odier to consist chiefly of Chitine, a substance which constitutes the basis of the elytra of insects, and which differs from gelatine and coagulated albumen in its insolubility in boiling water or caustic potash.
The crusts of crabs and lobsters, though they exhibit various tints of colours when the animals are alive, or not acted upon by heat, yet after death they are suddenly changed to a red colour when subjected to the action of boiling water. This change appears to take place in consequence of the decomposition of a substance, which leaves in a free state a red coloured oil, which, though insoluble in cold or boiling water, is soluble in cold alcohol and sulphuric ether. Besides these singular ingredients in the crust, M. Chevreul has detected the presence of iodine.
The moulting of the crustacea, or the casting of their skins, takes place at seasons of the year differing somewhat according to the species. During this period of their weakness, when their crusts are yet soft, and incapable of yielding the requisite protection which in a mature state they afford, these animals become the prey of their numerous foes. The evil is in part guarded against by their retirement to places of shelter and concealment. In their moulting state we have taken half a dozen of the Lithodes maia from the stomach of a codfish, a place to which they would not have been conveyed had their crusts acquired their mature hardness.
SECT. II.—ORGANS OF MOTION.
The residence to which the crustacea are in a great measure restricted, confines their motions to swimming, walking, and leaping. For the purpose of executing the first of these motions, the feet are constructed with flattened joints, which present a considerable surface of resistance to the surrounding medium; and their expanse is greatly enlarged by the fringe of stiff hairs by which they are in general surrounded, as in Portunus pilifer, figured in PLATE CLXXX. The gravity of their bodies being considerably greater than the surrounding medium, their suspension and motion in the water can only be effected by the active use of their limbs, while their progress is usually irregular. In walking, the crustacea, in general, advance sidewise, owing to the motions of the limbs being perpendicular to the mesial plane, instead of parallel to its direction, as in many other animals. Their progress, even laterally, is irregular and constrained. Indeed they appear better adapted to move over an uneven surface, by a process resembling climbing, than by ordinary walking. They mount with ease along the leaves of the sea-weeds, and the stems of the zoophytes, in search of their prey.
When pursued by their foes, or even in the execution of their ordinary purposes, many species are capable of leaping with considerable facility. This kind of motion they effect by suddenly unbending their tail or abdomen. In swimming they not unfrequently combine in their progress the leap with the employment of the feet as oars. In some species the kind of progressive motion is confined to walking or climbing; in others the motion is exclusively natatory.
The limbs of the crustacea consist of the five following parts: The hip is the medium of articulation between the limb and the body; and is divided into two parts; the coxa next the body, and the trochanter articulating with the thigh. These two joints are usually short, and nearly equal in size. The thigh or femur is usually the longest and the thickest of all the joints in the limb. The leg or tibia, though shorter than the thigh, frequently exceeds in dimensions, especially in length, the hip-joints. The tarsus is in general as large as the leg, and supports the last joint or claw. When the claw or unguis is placed on the extremity, or as a continuation of the tarsus, a simple leg is formed; but when placed laterally, so as to move and act in opposition to the extremity of the tarsus, it performs the function of a thumb; and the limb is said to have pincers, or to be didactyle.
The facility with which the crustacea can throw off their limbs when they are injured or detained, has often attracted the notice of the curious. The suture at which the parting takes place sustains but little loss by bleeding; and is soon able to exercise its reproductive functions. The new-formed limb, however, is smaller in all its dimensions, though consisting of the same number of parts, than the one thrown off; and it is not till after several moultings that the new limb acquires its mature size. The repairing power can more easily renew a whole limb than an individual joint, and is incapable of repairing a bruised or mutilated one. When the annual moulting of the crustacea is taken into consideration, this renewing process of the limbs will excite less surprise, though it still displays a striking feature in the economy of these animals; that in order to save their bodies from capture by their foes, they can cast off so readily the claws or legs which have been seized.
**SECT. III.—ORGANS OF SENSATION.**
The nervous system in the crustacea may be considered as exhibiting three different modifications. The first of these forms is displayed in the Podophthalmus, or those with moveable pedunculated eyes, in which the brain is formed above into four lobes, the two middle giving rise to the optic nerves. Below, four nerves take their rise, and proceed to the antennae, detaching in their course some filaments to the neighbouring parts. The posterior portions of the brain give rise to the two cords, which, in their course, supply the mandibles with filaments, and, after encircling the alimentary canal, unite in forming a medullary oval ring-like mass in the middle of the thorax, many times larger than the anterior ganglion or brain. From the circumference of this ring nerves proceed to the jaws and feet; and, retrally, a single nerve is sent to the tail. This arrangement prevails in the common crab. In the cray fish, the cords which proceed from the brain are shorter, and, after forming the collar, unite in an oblong ganglion; and supply the jaws with nerves. Leaving this ganglion, the two cords remain near each other, uniting at intervals to form five ganglia for the supply of the corresponding pairs of feet, and afterwards proceed into the tail, where they form six ganglia: the last of these masses gives rise to four nerves, which are distributed as radii to the scaly fins that terminate the tail. In the first condition of the nervous system the ganglia are reduced to two in number, by the union of all the ganglia behind the collar into a single medullary ring, a form well suited to the compact orbicular form of the body. In the last condition the lengthened form of the body draws out, as it were, the nervous cords, to form several ganglia, though these cords preserve a tendency to approximate throughout their whole extension.
The second form of the nervous system in the crustacea is exhibited in the Edriophthalma, or such as have sessile eyes. Among these animals the two cords of the nervous system, as in Talitrus, in proceeding from the brain, do not coalesce at any point, but form their ganglia separately, symmetrically, and in pairs. Though this modification of the nervous cords be so different from that which prevails in the former group, in the tendency of these cords to keep separate, there appears in Idotea and other genera a tendency in each cord to become accumulated or condensed towards the middle of its ordinary course, as in the common crab.
In the third form of the nervous system, occurring in the Entomostraca, the development is imperfect. In some the brain is in the form of a nearly transparent globe, the medullary cord double, with an enlargement at each of the numerous articulations of the body. In others the substomachal ganglion, though distinct, gives rise to filaments which, though representing the two cords, scarcely swell into ganglia in their course.
The eyes of the crustacea are either compound or simple. The compound eyes are sessile in one group, and little elevated above the common integuments. In another they occupy the extremity of a moveable peduncle, differing greatly in length according to the genera. In general the eyes are two in number, in some cases four, and in a few instances they seem to unite in one spot. The compound and simple eyes occur in different animals, though in other cases in the same individual both kinds of eyes exist.
The organs of hearing among the crustacea generally may be considered as in a great measure obliterated. Among the decapodous podophthalmia, however, distinct traces of this sense may be observed in a perforated cavity, situate at the base of each of the exterior tentacula, filled with a watery fluid, and supplied with auditory nerves. The sense of smell is inferred more from the habits of the species than from their structure, and a similar remark is applicable to the function of taste. The organs of touch, however, are very fully developed. The number of tentacula they possess, together with numerous other external organs, readily give information of the presence of objects, and likewise of their shape. The eye and the touch are indeed the two senses which occur in this class in the fullest development. The taste and smell perhaps come next in order.
**SECT. IV.—ORGANS OF DIGESTION.**
The organs exhibited by the crustacea, which assist in the capture of their prey, are of various kinds. The organs of motion are extremely well adapted for preserving within the reach of the mouth the object which is to serve as food, and the organs of the mouth exhibit a remarkable degree of complexity. In the decapodous crustacea an upper lip or labrum may be observed, covering a pair of mandibles with their palpi. Underneath these may be found the first pair of maxillae, similar in many respects to those belonging to insects. The second pair of maxillae, which follow, appear to occupy the place of the under lip or labium of insects, within which is a tongue, but imperfectly developed, and usually bifid. These organs of the mouth are covered by three pair of auxiliary maxillae, which are regarded as corresponding with the three pair of feet in insects, an analogical inference which receives considerable countenance from the circumstance that the number of legs increase in certain genera, in proportion as the auxiliary maxillae diminish in number, these undergoing modifications fitting them for their new functions. The ordinary food of the crustacea appears to be either minute zoophytes and molluscous animals, or dead animal matter. In some species a preference appears to be given to vegetable food; and one crab, the Birgus latro, is celebrated for climbing trees for the purpose of stealing or rather eating cocoon nuts. To entice their prey, or conceal their own approach, a few crustacea, as the Isechii and Macropodidae, cut portions of fuci and corallines, which they cement to their bodies, and in this grotesque style of dress effectually hide their true forms. It is not improbable that in this garb they can transport themselves more readily from one place to another, it being practised chiefly by those species which are bad swimmers. The dead substances on which so many of the crustacea feed are probably discovered by the help of their organs of smell; and so quickly do they detect substances suited to their taste, that the lobster creeds and crab pots, when suitably baited, seldom fail to allure their victims.
The masticating organs are succeeded by a short gullet opening into a stomach more or less complicated, with cartilaginous bands, in many genera furnished with osseous teeth, to complete the reduction of the solid food which the mouth had been unable to bruise. On each side of the stomach a round flat calcareous stone is found previous to the moulting, which serves as a store of earthy matter for the renewal of the shell. These stones, vulgarly known by the name of crabs' eyes, were long highly esteemed for their medicinal virtues as absorbents. The intestine, in the decapodous crustacea, has a valvular enlargement with an attached caecum. In the other groups the intestine is simple, proceeding directly to its terminal opening under the tail.
**SECT. V.—ORGANS OF ASSIMILATION**
The nutritive fluids derived from the contents of the stomach and intestines, are conveyed, in the decapodous crustacea, to a pulmonic ventricle, situated dorsally and retrally in reference to the stomach. From this vessel one or two principal arteries have their origin, and transmit the blood by as many subdivisions as there are branchiae. After the process of aeration has been effected, the blood is returned to a heart, placed ventrally or below the intestine, forming in this manner a nearly complete double circulation.
Among the Entomostraca the heart appears to be single, and to approach in its character the dorsal vessel of insects.
The organs of respiration are of three kinds. Among the decapoda they are plumose pyramids, situate at the base of the feet, covered by the shell, and watered by a current which enters by an aperture from behind the feet, and escapes by an opening near the mouth. Two pair belong to the auxiliary maxillae, and five pair to the true feet, in the true crabs; while among the lobsters the branchiae belong to the four anterior feet, and the first auxiliary jaws have four in each group, making twenty-two branchiae in all, on each side, instead of fourteen as in the former group.
In another group of crustacea the branchiae are not concealed, as in those already noticed, but are external, appearing under various forms, attached to or occupying the place of natatory feet. In a third the branchiae are reduced to vascular scales, differing greatly in the degree of exposure or protection.
The liver occupies a prominent place, being very fully developed. In the decapoda it is placed ventrally to the stomach, large in its size, indeterminate in its form, and of a yellow colour.
Among the peculiar secretions of the crustacea, the singular property of emitting light, which some species possess, merits some notice. On the passage from Madeira Crustacea to Rio de Janeiro, the sea was observed by Sir Joseph Banks to be unusually luminous, flashing in many parts like lightning. He directed some of the water to be hauled up, in which he discovered two kinds of animals that occasioned the phenomenon; the one a crustaceous insect, which he called Cancer fulgens. In form it bears some resemblance to the common shrimp. It is however considerably less, and the legs are furnished with numerous sets. The light of this animal, which is very brilliant, appears to issue from every part of the body. The light is otherwise distributed in another crustacean animal, observed by Captain Tuckey in the Gulf of Guinea. The luminous property resided in the brain, which, when the animal was at rest, resembled a most brilliant amethyst about the size of a large pin head; and from this there darted, when the animal moved, flashes of a brilliant silvery light.
**SECT. VI.—ORGANS OF REPRODUCTION**
With the exception of some entomostracous genera, the sexes are distinct among the crustacea in different individuals. The males abound in some of the genera, in others the females occur in apparent excess. In the decapods the seminal organs are situate between the stomach and heart, towards the centre of the body, and send forth two vasa deferentia to the two external organs, which may be observed behind the fifth pair of feet. The ovaria also occupy a central position, and each sends out a tortuous oviduct, opening in the corresponding third sternal plates at the base of the third pair of feet. With the edriophthalia, in general, the sexual organs are placed under the tail. Among the Entomostraca these organs vary greatly in the different groups, and in some genera the sexes appear to be united; and among the Daphnidea a single intercourse with the male suffices to render prolific seven or eight generations.
The evolution of the egg takes place in several of the Entomostraca within the body of the mother, but in general the hatching is external. In the latter case the eggs, often amounting to a great number, are attached by a glutinous cement to certain filaments under the tail, or to certain plates with which the branchiae are also connected. In other cases, as Cyclops quadridentatus, the uteri are external sacs.
When newly hatched, the young of some of the Entomostraca nearly resemble the parent, while in others, the difference is so great as to have given rise to spurious genera. In the more perfect groups, as among the edriophthalia, the young are deficient in the number of segments of their body and corresponding number of feet, which they acquire by degrees. Among the decapoda it was long considered that they issued forth from the egg in a perfect state. The interesting "Zoological Researches" of Mr Thompson have, however, unfolded more correct views, and demonstrated the existence of a metamorphosis so great, that in the first stages of their growth they have been constituted into particular genera, of which zea is the type. In this condition the young are essentially natalory, with cleft members, which change into simple feet, solely adapted for crawling. Mr Thompson has traced this change in many decapodous genera. In the genus Mysis the change from the young to the adult state is effected by a gradual and successive development of parts. According to the observations of Rathke, the young crawfish, when it bursts the egg, has the external organs formed, but in a soft state, and advances to maturity by gradual development.
With the observations of Mr Thompson in view, con- Crustacea trusted with those of Rathke, it seems desirable that the early life of the crustacea were studied with care, and the facts established subjected to the process of cautious generalization.
The duration of the crustacea has not been investigated with care. Among the Entomostraca the individuals seem to perish with the close of the season in which they had their birth, leaving behind them eggs, to be hatched in the following spring, as preservatives of the race. In the higher groups, especially the decapoda, life is maintained during a succession of years. When moulting, they are an easy prey to their foes; and when their shell has acquired its hardness, the seeds and eggs of a variety of parasitical plants and animals adhere to it, locomotion is retarded, and starvation follows.
In the distribution of the crustacea, the greater number of species live in the sea, a few in fresh-water lakes and rivers, and fewer still on the land. These last, however, visit the sea periodically to deposit their spawn, and at all times can preserve their gills in a moist state, to enable them to effect the process of aeration of the blood. Some lodge in holes excavated in the mud, while others seek protection in a deserted univalve shell, requiring such a covering to their tender and soft posterior extremity.
The species which have become extinct are few in number, and chiefly confined to the newer series of rocks; if we except the anomalous group of Trilobites, afterwards to be taken notice of. In the rocks of the saliferous epoch few traces of crustaceous organisms occur. During the cretaceous epoch, however, or at the period when the various deposits of oolites, flas, and chalk were in the course of formation, several crustaceous animals appear to have flourished, the remains of which now occur imbedded in these rocks. The species chiefly belong to the Macrourous tribe, approaching generally the shrimps in character. In the strata of the penult epoch, or those which are interposed between the chalk and the sedimentary masses of a comparatively modern date, the remains of crabs and lobsters occur pretty generally distributed. In the rock well known under the denomination London Clay, they have been found imbedded in considerable numbers. The cause of the extinction of these crustaceans species, at these different periods, must be sought for in those physical alterations which took place on the globe, by which the very materials brought into the seas and lakes by rivers, to form stratified matter, at these distinct eras, presented altered characters. Such changes rendered the seas unfit to support one race of beings, while it became a suitable abode for those which were destined to succeed.
The crustacea are extensively employed for dietetical purposes. They are eagerly sought after on many parts of our rocky coast, and traps for their capture are in very general use. This kind of fishery, as being of a littoral kind, is chiefly prosecuted by the more aged of our maritime population, who have become unfit for employment where greater vigour is required. These fishers make themselves familiar with the haunts as well as the general economy of their game, and the kind of garbage which constitutes the most enticing bait. As food, the crustacea are used either plainly boiled, or dressed with various condiments, sometimes also marinated with vinegar.
CHAPTER II.
SYSTEMATICAL ARRANGEMENT OF THE CRUSTACEA.
The species known to Linnaeus were few in number, and were distributed into the four genera, Cancer, Oniscus, Monoculus, and Phalangium. In the genus Cancer the celebrated Swede introduced a number of sections or subdivisions which succeeding authors have raised to the rank of orders or genera. Fabricius enlarged the number of genera of crustaceous animals, assigning to them, at the same time, more precise distinguishing characters. In this field of improvement he was followed by Latreille, Lamarck, Dumeril, and Leach. The last-mentioned naturalist, by great zeal and labour, has not only added many new species to the class, but has succeeded in unfolding many new characters, and giving to the generic marks a degree of precision which they did not previously possess.
The various groups into which different naturalists have divided the crustacea resemble one another more in their composition than in the terms by which they have been designated. If we assume the aerating organs as the basis of arrangement, we shall find, correlative with these, certain distinguishing features in the circulation and in the nervous system, as well as in the digestive and locomotive organs. Such considerations appear to have induced Lamarck to distribute the crustacea into two orders, which he has termed Homobranchia and Heterobranchia, the former having a solid shield concealing the branchia, which are placed on the sides at the base of the feet and auxiliary jaws, while the aerating organs of the latter never occur in the same position, but are variously placed in the different subordinate groups. The organization of the first order is extensively developed, and furnishes well-marked features of discrimination, separating it from the second division, which includes animals of more simple structure.
ORDER I.—HOMOBRANCHIA.
The crustaceous animals of this order have the body covered by a continuous shell, which contains the viscera, and a tail which serves the threefold purpose of giving passage to the extremity of the intestinal canal, of acting as an organ of progressive motion, and of retaining the eggs by means of its fringed appendages. The thorax and abdomen, as well as the head, may be considered as united. The eyes are two in number, and compound, lodged in cavities in the antenal margin of the shield, and are supported on moveable peduncles, which vary greatly in their length in the different groups. The peculiar footstalks of the eye have given rise to the term Podophthalmus, by which in some systems they are denominated. The antennae are four in number, the intermediate pair being divided into filaments. The mouth occurs in this group in its most complicated form, with three pair of auxiliary jaws furnished with palpi. The legs are ten in number, the first pair usually of a large size and formed into pincers, the rest terminating in simple claws. In some cases, however, the second and third pairs are also formed into pincers; and in some of the genera the posterior pair have compressed joints as ears for swimming. The regularity in the number of feet in this group has suggested the term Decapoda, by which they are denominated by Latreille. In the structure of the tail the animals of this division exhibit very remarkable differences. In one tribe the tail consists of but few joints, the last one being simple, the whole shorter than the body, and usually folded up under the shield in a cavity for its reception. This shortness of the tail has suggested the title Brachyura. In the other tribe the tail is usually exposed, about the length of the body, and furnished with plates at the extremity. The second form of the tail has given rise to the term Macroura, by which they are known.
Tribe Brachyura.
In this division of crustaceous animals the body is usually of an orbicular or compact form, and this condition, joined with the imperfect development of the tail, restricts the progress of the species to crawling rather than swim- ming. The animals here referred to are well known under the familiar name crabs. The sexes are readily distin- guished by the form of the tail, which in the male is narrow and short, while in the female it is proportionally of much greater dimensions.
Among those animals of this tribe which exhibit the more ordinary forms, there is one family characterized in a particular manner by the structure of the feet being adapted to swimming. In general with such the body is broad, rather short, rounded anteriorly, and the margin spin- ous. With the exception of the pincers, which are fitted for grasping, all the remaining feet in some genera are form- ed for swimming, while in others the flattened form of the joint is restricted to the hinder pair. The facility with which these animals can transport themselves permits a wider range of motion than the other species, which are chiefly confined to the shore. In a foreign group, includ- ing the genera Podophthalmus and Lupa, the peduncles of the eyes are remarkably produced, and received into a groove on each side. In the other genera the eyes have short stalks lodged in oval cavities. The genus Portunus contains many species which are common on our shores, but seldom applied to any useful purpose.
A second family of crustaceous brachyuri, denominated Arcata, have the joints and claws of the feet rounded and conical. The anterior margin of the shield is semi- circular and full, while the posterior is contracted and truncated. Like the preceding family, the tail of the males in general has only five joints. They chiefly inhab- it our rocky shores, and are eagerly sought after. It is in this group that the cancer pagurus, or common crab, oc- cupies its place in the system generically characterized by the large basilar joints of the exterior antennae, and economically employed as an esteemed article of food.
The family Quadrilatera, as the name implies, have a more or less quadrangular form, in some species approach- ing to heart-shaped, with the front inclined, forming a sort of hood, and truncated. The tail in both sexes consists of seven distinct fragments. Though in these points of re- semblance the genera of this family stand related to one another, yet in their habits or physical distribution they exhibit the most striking differences. Among those which inhabit the sea, the Gonoplex angulata, a species not un- common on the southern and western coasts, lives in ex- cavations formed in the hardened mud, open at both ex- tremities; the Pinnothorax, a genus containing several species, take up their abode in living bivalve shells, such as mussels, penae, and oysters. By some of the ancients these crabs were considered as acting as sentinels; by others, as co-operating for interested purposes.
Thus fed by mutual aid, the friendly pair Divide their gains, and all the plunder share.
A few of the species of this group, belonging to the genus Thelphusa, are restricted to fresh water, from which however they can withdraw themselves for a while to the air. The Thelphusa fluviatilis, figured at Plate CLXXX., found in Italy, the Levant, and Egypt, is esteemed as food, and occupies a prominent figure on ancient Greek medals. When the attention of the economists of this country shall have been suitably fixed on the importance of peopling our present watery wastes, including lakes and rivers, with animals of ornament and use, it appears probable that this species of crab will speedily be naturalized, its habits fur- nishing every facility for its transportation.
A third group in this family are capable of inhabiting the land, burrowing in holes, and feeding on all sorts of garbage. They are chiefly confined to the equatorial re-
gions. As an example, see the Gecarcinus ruricola, figured Crustacea on Plate CLXXX.
A fourth family, denominated Orbiculata, exhibit a round- ed shell, usually solid and smooth, the peduncles of the eyes short, the pincers of unequal length, according to the sex; the region of the mouth contracted at its upper ex- tremity, with the third joint of the outer auxiliary feet in the form of a lengthened triangle. The Corystes casse- lanus, figured Plate CLXXX., belongs to this group—a species which is very common in the sandy bays of the country, and frequently cast ashore after storms.
The group which has been denominated Trigona is more distinctly marked than some of the preceding fami- lies. The body is full and rounded behind, and produced and pointed before, and in many species armed with strong spines. The legs are usually long and slender, and hence the name of sea spiders, by which they are familiarly known. The region of the mouth is nearly square. To some of the species, as Maia squinado, a susceptibility to the charms of music and extraordinary sagacity have been attributed. Another species, Hyas araneus, is de- tested by the oyster fishers, as injurious to the young spawn.
The two remaining groups of brachyuri, which may be denominated Heteropoda; from the condition of their legs being peculiar, are capable of division into two families. The first, termed Cryptopoda, have the shield semicircular or triangular. The feet, with the exception of the pincers, are capable, when at rest, of withdrawing into a marginal cavity of the shield. In the genus Calappa, the pincers are elevated into a crest above, and folded in the anterior margin so as to close in the front of the shield. The spe- cies are natives of equatorial seas.
The other group, termed Notopoda, is distinguished by the last pair of feet having their origin above the general margin of the shield. This gives to these organs a dorsal rather than a ventral aspect. The species are few in number, and in general strangers to the northern seas, though one species of the genus Dromia has been recent- ly added to the British fauna.
2. Tribe Macroura.
The crustaceous animals belonging to this division, fa- miliarly known as lobsters or shrimps, have the body more produced than in the preceding. The tail is as long, or longer, than the body, exposed or simply recurved at the extremity. It is composed of seven segments, with ap- pendages at the extremity, forming a fin. In their mo- tions they are more agile than those of the preceding group, and as their organs are of a natatory character, the species are restricted in a great measure to the waters.
In the first division of this tribe, including the species of more perfect organization, the first pair of feet at least are in the form of pincers, the remainder not bifid or na- tatory. The lateral appendages of the tail are so placed as to form a fan-shaped fin.
In the first subdivision of these crustacea, denominated by Latreille Carides, the exterior antennae are placed more ventrally than in the intermediate pair, and have their pe- duncles covered by a large scale or plate. The snout pro- jects, and is frequently spinous, while the body is usually much incurred. There are five pairs of caudal feet. Some of the species are eagerly sought after as food, as the common shrimp, Crangon vulgaris, along with another species, Palemon serratus or prawn.
The second subdivision, including those species having the antennae placed on the same line, readily admit of dis- tribution into three families. The first, including the ge- nus Scyllarus, has the pincers imperfect, and the external antennae destitute of a stalk, with the basilar joints dilated, Crustacea, forming a horizontal crest on each side. It may be considered as an equatorial group.
The second family exhibits equally imperfect pincers, but the external antennae are largely developed. The *Palinurus vulgaris*, or thorny lobster, sometimes also termed cray-fish, is a well-known example in the markets of London and Paris.
The third family, of which the common lobster may be regarded as the type, has the first pair of legs in the form of pincers, and in some of the other genera the second, and even the third pairs, are didactyle. The group contains a few fresh-water species, among which the *Astacus fluviatilis* or craw-fish may be mentioned, as inhabiting many of the rivers of England, in the clayey banks of which the individuals form their holes.
The second division of *Macroura* exhibits certain anomalous characters, which separate them from the more perfect and regular group to which we have made reference. The first subdivision comprehends the hermits or soldier crabs. These are distinguished by the tail not being in the form of a fin, and the diminished size of the posterior legs. In one group all the upper parts are crustaceous and protected; but in another, a portion of the thorax and the tail being membranaceous, they are exposed to greater danger. This is guarded against in a remarkable manner. Many of the species inhabit deserted univalve shells, introducing their short tail into the spiral cavity of the shell, their thorax filling the last whorl. The pincers, which are in some measure protected by the pillar of the shell, are the least in size. These crustacea move easily from place to place in their assumed abode, and shift into a shell of larger dimensions as their own bodies increase in size. The *Pagurus Bernhardus* is common on all our shores, and is figured at Plate CLXXX.
Another species, the *Pagurus Diogenes*, common in Jamaica, for example, may be viewed as a terrestrial species.
The second subdivision, denominated *Fissipedes*, are destitute of pincers, while the feet have an appendage externally, issuing near the base, giving them a natatory character. Instead of the eggs being carried by the false feet under the tail, they adhere to the base of the thoracic feet; a circumstance which has given rise to the denomination opossum shrimps.
**ORDER II.—HETEROBRANCHIA.**
The crustaceous animals of this order exhibit characters more varied and anomalous than those of the preceding order. The branchiae are external when they consist of imbricated plumes, or in the form of aerating pouches, but never placed, as in the preceding order, under the shield, but either on the under side of the belly or tail, and unconnected with the auxiliary jaws. They are of small size when compared with those already noticed.
The animals of this order may be divided into two tribes, in the first of which should be placed those which have their mandibles bearing palpi, and into the second those having mandibles destitute of such appendages.
Among the crustaceous animals of the first tribe with mandibles furnished with palpi, the *Stomatopoda* seem entitled to hold the first rank. In the pedunculated form of their eyes they resemble the animals of the order homobranchia. The branchiae are exposed, and attached to the five pair of appendages under the tail, which act as natatory organs. The covering is usually soft, and even transparent. In general the species of this family inhabit the tropical seas. The *Squilla mantis*, however, is a native of the Mediterranean; and the *Alima hyalina*, from the west coast of Africa, is figured in Plate CLXXX.
With the second family of palpigerous crustacea, denominated *Amphipoda*, the group commences which has been termed *Edriophthalma*, having sessile eyes, all those groups previously referred to having their eyes seated on a peduncle. The Amphipoda have the body usually compressed and incurved. The first pair of feet, corresponding to the second pair of auxiliary jaws in the decapod crustacea, are attached to the first segment of the body. The branchiae appear to be constituted by the hairs on the divided natatory appendages under the tail. Some of the species occur in rivers and streams, others attach themselves to the sea shore. They swim on their side, and many species leap with facility, by the sudden unbending of their body. They prey upon dead animal matter, and constitute a favourite article of food for the birds which frequent the shore.
In the crustacea which follow, the mandibles are destitute of palpi. The family denominated *Lemmalipidae* are distinguished by the absence of the tail, the last pair of legs being inserted near the extremity of the body. The anterior pair of feet are attached to the head; these, together with the following pair, which are the largest, end in pincers. The tentacula are four in number. At the base of the third and fourth pairs of feet, in some cases also of the second pair, are vestigial appendages. The third and fourth pairs of feet are in some species greatly reduced in their organization. The species of some of the genera dwell among fuci and zoophytes, while some *Cyamus* attach themselves to the bodies of cetaceous animals, and are on that account known to sailors under the denomination of whale lice. The *Cyamus ceti* is figured on Plate CLXXXI.
The *Isopoda* differ from the preceding group, in the anterior feet not being attached to the head, but, like the following pairs, connected with a distinct segment of the body. Leaf-like appendages, or vesicles, occur under the tail, usually inclosed or covered by lateral plates. The habits of this group exhibit very remarkable differences. The *Euparoides*, having neither eyes nor antennae, are parasitical on the prawn.
Among the *Cymothoidea*, the *Limnoria terebrans*, though of small size, is well known as peculiarly destructive to beams of timber employed in the construction of sea harbours.
The peculiar character of this animal was first ascertained by that eminent engineer Mr Stevenson, during his observations connected with the construction of the Bell Rock light-house. It excavates its cylindrical hole for its dwelling, and propagates so rapidly that in a few years submerged timber is rendered useless. So destructive indeed is this small animal, that Norway logs laid down at the Bell Rock to support temporary railways in 1807, were found, when lifted in 1811, to have been reduced by its ravages from ten inches square to seven inches, or at the rate of about an inch in the year. Mr Stevenson, in order to observe more particularly the ravages of this destructive animal, fixed down on the Bell Rock in 1814 specimens of teak wood, oak, black birch, Memel and Norway fir timber. The only specimen which remained unperforate till 1820 was the teak wood; the rest were almost entirely destroyed in the course of two or three years. Sheathing with copper the beams thus employed in marine architecture, seems to be the only preventive of the injuries of this small but successful destroyer.
The *Idoteidae* have habits, as marine species, differing little from the Amphipoda. One species, figured at Plate CLXXXI, has been denominated, from its locality, *Astacilla Baffini*, a native of Baffin's Bay. The genus to which it belongs was instituted by the Rev. Charles Corderer of Banff in 1784, for the reception of a British species which has been denominated *Astacilla longicornis*. The same species was afterwards described in Sowerby's *British Miscellany* in 1805, as a new species, under the title *Oniscus lompiensis*. In 1825 it was produced by Dr Johnston of Berwick, not only as a new species, but as constituting a new genus, to which the name *Lestia lacertosa* was assigned. The other species, *Astacilla Baffini*, was collected by Captain Parry among the marine productions of the Arctic seas; and figured and described by Captain Sabine, in the appendix to the First Voyage, as *Motus Baffini*. Overlooking the observations of our arctic voyagers, M. Latreille, in the fourth volume of the *Regne Animal* de Cuvier, revived the arctic species under the new title *Arctarius tuberculatus*.
In those denominated *Asselloidea*, the *Assellus aquaticus* occupies a distinguished place. It is a fresh-water species.
The Oniscidea constitute the group. They are terrestrial, requiring, however, damp air, to enable the branchiae to exercise their functions. They are denominated in England woodlice, in Scotland slaters.
The last crustacean group of the *Heterobranchia* which we have to notice have been denominated *Entomostraca*, or *Branchiopoda*. They have corneous integuments, with a shield in one piece, or divided like a bivalve shell. Hence the origin of the first of the epithets by which they have been characterized. The branchiae are in the form of hairs or feathers, forming a part of the feet. In general these crustacea are of small size, and may be denominated microscopic. The fry of some of the decapodous crustacea have in this group appeared as distinct genera; while the fry of species belonging to the group have been raised to the rank of genera.
The first great division of the Entomostraca includes the animals of which the genus *Monoculus* of Linnaeus consists. They are distinguished by their simple natatory feet, the eyes united, the antennae generally four in number, the mouth composed of a labrum, two mandibles, a tongue, and one or two pair of jaws. The head is incorporated with the thorax.
In one family, the *Lephyrops*, the feet do not exceed ten, with joints more or less cylindrical. The *Cyclops quadricornis*, to be found in almost every fresh-water pool, is an interesting example of the group. When kept in a phial, it multiplies with amazing rapidity, and forms a pleasing object for the microscope. The females may be readily distinguished by the bags on each side of the tail, as represented in Plate CLXXXI., containing the eggs. In another family, in which the body is inclosed in a bivalve shield, the individuals of many species are to be found in ditches in vast numbers, and form very pleasing objects of observation. Among these the peculiar markings of the *Cypris ornata*, figured at Plate CLXXXII., seldom fail to arrest the attention.
In the second family, denominated by Latreille *Phyllopida*, the number of feet exceed twenty, and their joints, especially of the posterior ones, are flattened in the form of ciliated leaves. The eyes are two in number, and in some instances pedunculated. The *Artemia salina* occupies a prominent place in this group. It is commonly known by the name of Lymington shrimp, or brine worm. The Rev. Mr Rackett has furnished the following interesting notice of this curious animal:
"Myrids of these animalcula are to be found in the salterns at Lymington, in the open tanks or reservoirs, where the brine is deposited previous to the boiling. It attains the desired strength by evaporation, from exposure to the sun and air, in about a fortnight. A pint contains about a quarter of a pound of salt; and this concentrated solution instantly destroys most other marine animals.
"These tanks are called clearers, as the liquor becomes clear in them; an effect which the workmen attribute in some degree to the rapid and continual motion of the Crustacea-brine-worm, or to the particles which cloud the liquor serving for its food; but this is mere conjecture. So strongly persuaded, however, are the workmen of this fact, that they are accustomed to transport a few of the worms from another saltern, if they do not appear at their own. They increase astonishingly in the course of a few days."
It is observable that the brine-worm is never found in the salterns, where the brine is made by the admission of sea-water during the summer, and which are emptied every fortnight; but only in the pits and reservoirs, where it is deposited after it is taken out of the pans, and where some of the liquor constantly remains. When it becomes much diluted with rain-water, from October till May (during which time the manufacture is at a stand), a few only of the worms are visible; but at the approach of summer young ones appear in great numbers."
Another species, the *Branchipus stagnalis*, figured in Plate CLXXXII., greatly magnified, is found in ditches of soft standing water, in which they swim constantly on their backs, keeping themselves suspended by the vibrations of their numerous fins, and moving forwards by giving a sudden spring with their tails. They delight much in the sunshine, during which they appear near the surface of the water. On the least disturbance they start in the manner of small fish, and endeavour to secrete themselves by diving in the soft mud.
A third species of the group, the *Lepidura prolongata*, greatly resembles the preceding, in being found in stagnant marshes, usually in great numbers, making their appearance in certain years in those places to which for a time they had been strangers.
The second great division of the Entomostraca, denominated by Latreille *Paciopoda*, differ from the preceding in the ambulatory or prehensile character of the anterior feet, and the branchial or natatory forms of the posterior ones. They are likewise destitute of mandibles, and even jaws, the mouth being suited for soft food. They live, in general, as parasites on the bodies of fishes, feeding on their substance. The *Caligus curtus* feeds on the gills of the salmon; the *Anthosoma Smithii*, figured in Plate CLXXXII., adheres to the shark.
Besides these more ordinary groups of crustacean animals, to which a general reference has been made, there are yet two others to which we shall merely allude. The *Trilobites* occur only in a fossil state. They make a near approach to the onisci. But even in the best preserved specimens the existence of feet has not been satisfactorily determined. By the labours of Bronnart, Desmarest, and others, the permanency and difference of certain forms have been ascertained, so as to justify the establishment of several genera in the group, and enable geologists to employ more definite language.
The *Cirripedes*, including the shells which constituted the Linnean genus *Lepas*, make a near approach to the crustacea in many respects, especially to the Entomostraca. But in an article of this kind it would be unprofitable to give the detailed view which would be requisite for the purpose of establishing the relationship, by the exhibition of the analogies which exist.
The crustacea are in general very easily preserved for the cabinet. Their members readily relax after being dried, and admit of being set according to any particular attitude. The species which inhabit the sea require to be steeped for several hours in fresh water before they are dried. If this precaution be omitted, the salt water which adheres to the several parts, especially the ligaments, keeps them damp, and occasions mouldiness and decomposition.
In studying the crustacea generally, the following works may be consulted with advantage:—Latreille, Histoire Naturelle générale et particulière des Crustaces, et des Insectes; Lamarck, Histoire Naturelle des Animaux sans Vertébres; Cuvier, Regne Animal; Desmarest, Considérations générales sur la classe des Crustaces.
In the examination of the British species, useful help may be obtained from the British Zoology of Pennant. The Entomologist's Useful Compendium, by Samouelle, contains a compilation of Dr Leach's views of arrangement of the crustacea, and will yield much assistance, especially if Dr Leach's splendid work, Malacostraca Podophthalmia Britanniae, be at the same time accessible.