For many years the influence of the artificial method of Linnaeus, in botany, had been successfully assailed in France ; and the followers of the natural method at length became so numerous as to secure it a favourable reception. Various efforts were likewise made to pro mote its introduction into the science of zoology. This was at length triumphantly effected, in regard to mollus cous animals, by M. G. Cuvier. Other enquirers, it is true, largely contributed to the accomplishment of this desirable end : three of whom, distinguished by their patient industry, deserve to be particularly enumerated, Adamson, Muller, and Poli. The former paved the way to the modern arrangements, in his Historic Xaturellc do Senegal. Muller explored the molluscous animals of Denmark, and published the result of his observations in the Zoologia Danica, and his Vermium Terrestrial): et Historia. Poli, directing his attention to the structure of the animals inhabiting the multivalve and bivalve shells, published his laborious observations in his Testacca utriusque Sicili r eorunzyue Historia et Anatomic. Many other authors might be enumerated, who have likewise contributed to extend our knowledge cf the structure of molluscous animals, as Lister, Monro, and Home ; but as we shall have occasion afterwards to advert to their labours, it is unnecessary to enter into further details.
The first efforts of Cuvicr to illustrate this depart ment of zoology, though necessarily imperfect, excited the attention of naturalists to the subject, and were the preluee to those mighty achievements, more recently made knonn to the world in his various papers inserted in the ..4nnales du Museum, and lepublished under the titlt Memoires pour servir a l' Histoire et a I 'Anatomic des Mollusques, in one volume quarto, Paris, 1816. These papers may be considered as models of minute and ac curate research, pet spicuous description, and candid criticism, and merit the careful and frequent perusal of the student of molluseous animals. In the second volume of a more recent work by the same author, entitled Le Rkne Animal destribue d'apres son Organization, Paris, 1817, he has given a synoptical view of the subject, which we intend to make the basis of the following article.
As a preliminary step to an examination of the divi sions of molluscous animals, we shall take a very gene ral view of their structure and functions, reserving the more minute details to be given under the different groups into which they have been arranged. Indect:, the several organs, as they appear in the different tribes, are so variously modified, that few common properties can he enumerated. In this section, however, we shall be able to mark the course to be observed in the illustra tion of the subordinate divisions, endeavouring, at the same time, to avoid saying any thing which will again require to be repeated.
I. General Form.—The molluscous animals exhibit very remarkable differences in their form, and the num ber and position of their external members. Neither
head nor foot can be observed in some, the principal organs being enclosed in a bag pierced with apertures for the entrance of the food and egress of the excre mentitious matter. In some, whose exterior is still remarkably simple, cuticular elongations, termed tenta cula, surround the mouth, and a foot, or i•.s•rumcnt of motion, may likewise be perceived. This last is in some free at one extremity, in others attached to the body throughout its whole length. In many species there is a head, not, however, analogous to that mem ber in the vertebral animals, and containing the brain and organs of the senses, but distinguished as the ante rior extremity of the body, separated from the back by a slight groove, and containing the mouth and tentacula.
In many of the animals of this division, the different members of the body are in pairs, and are arranged, in reference to a menial plane, into right and left. In some, part of whose organs respect a menial plane, other parts are single, or in unequal numbers. In other species, the organs, which are not in pairs, arc arranged round a central axis, and give to the external form a radiated appearance. But these characters are exceed ingly variable and uncertain, as marking the limits of particular tribes ; since, in different parts of the same animal, modifications of these forms may be readily dis tinguished.
2. Cutaneous System.--The skin of molluscous ani mals is more simple in its structure than the same or gan in the vertebral animals. The cuticle is here very distinct ; and, as in other classes, it is thick and coarse, where much exposed, but thin and delicate in its tex ture, where it lines the internal cavities. A mucous web may be detected in the cuttle fish and slug, but of great tenuity. The corium is destitute of a villous surface ; and on its central aspect it is so intimately united to cellular substance, that its fibrous structure can scarcely be distinguished. The witscukr web may, in general, be readily perceived. Its fibres proceed in various directions, according to the kind of motion to be executed, and extend or corrugate the skin at pleasure.
The appendices of the skin in this class of animals ought to be carefully studied, as they furnish the most obvious marks for distinguishing species, and for con structing divisions in their systematic arrangement. The appendices of the cuticle are few in number, and perhaps ought to be considered as limited to hairs. These, in some species, invest the surface regularly and closely, and may be observed on those o hick live on land, as well as those which reside in water. In some cases these hairs are as it were united, to form con,intious crusts or ridges. These hairs, as well as the cuticle, are liable to be worn off, and in some places can seldom be perceived, unless in early age.