MOLL

MOLL, a town of Belgium, province of Antwerp, on the right bank of the Moll-Nethe, 31 miles E. of Antwerp. The town contains several churches and schools, an hospital, and a prison. The principal manufactures carried on are,—brewing, distilling, dyeing, and the making of woollen stuffs, hats, leather, tobacco, and bricks. Pop. 4770.

Introduction. 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.¹

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 1794² and 1817,³ was the elimination therefrom of the order Cirripedia. In 1819 Straus Durckheim, in a memoir on the Structure and Affinities of an Entomostreacous Crustacean (Daphnia), published in the fifth volume of the Mémoires du Muséum, compared in detail the organization of a Cirriped (Pentelasma) with that of an Entomostreacous (Limnadia), and pointed out their close affinity; which affinity was, two years after, insisted upon and illustrated by the correspondences of structure between Pentelasma and Daphnia, by Mr W. S. Macleay, in his Horæ Entomologica, 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 larvæ of a small sessile barnacle (Balanus pusillus), which he showed to undergo a very remarkable metamorphosis. He captured their larvæ 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 larvæ 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 Cymothoe, 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,⁴ 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. Souleyet in 1852.

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-

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 Heterogangliata as above defined.