| OCR Text |
Show 304 MOLLUSCA. Mollusca. Most generally, however, it becomes so much developed, that the contracted animal finds sijelter beneath it· it is then termed a shell, and the animal is said to be testaceous; the epidermis which covers it is thin, and sometimes desic. cated(l ). The variety in the form, colour, surface, substance and brilliancy of shells, is infinite; most of them are calcareous; some are horny, but they always consist of matters deposited in layers, or exuded from the skin under the epidermis like the mucous covering, nails, hairs, horns: scales, and even teeth. The tissue of shells differs according to the mode of this de· position, which is either in parallel laminre or in crowded ver. tical filaments. All the modes of mastication and deglutition are visible in the Mollusca; here the stomachs are simple, there multiple, and frequently provided with a peculiar armature; their in· testines are variously prolonged. They most generally have salivary glands, and always a large liver, but neither pancreas nor mesentery : several have secretions which are peculiar to them. They also present examples of all the modes of generation. Several of them possess the faculty of self-impregnation; others, although hermaphrodites, require a reciprocal coitus, while in many the sexes are separated. The first are vivipa· rous, and the others oviparous; the eggs of the latter are sometimes enveloped with a harder or softer shell, and some· times with a simple viscosity. These varieties of the digestive and generative processes are found in the same order, and sometimes in the same family. The Mollusca in general appear to be animals that are but slightly developed, possessed of but little industry, and which are only preserved by their fecundity and vital tenacity( I). (1) .until my labours on the subject were made public, the Testacea constituted a particular order; but there are so m:.1.ny insensible transitions from the naked Mollusca lo the Testacea, and their natural divisions form such groups with each othet·, that this distinction can no longer exist. Besides this, there are several of the Testacea which are not Mollusca. MOLLUSCA. 305 Division of the Mollusca into Six Classes(!). The general form of the body of the Mollusca, being in proportion to the complication of their internal organization, indicates their natural division(2). The body of some resembles a sac open in front, containing the branchire, whence issues a well developed head crowned with long and strong fleshy productions, by means of which they crawl, and seize various objects. These we term the CEPHALOPODA. That of others is closed; the appendages of the head are either wanting or are extremely reduced; the principal organs of locomotion are two wings or membranous fins, situated on the sides of the neck, and which frequently support the branchial tissue. They constitute the PTEROPODA. Others again crawl by means of' a fleshy disk on their belly, sometimes, though rarely, compressed into a fin, and have almost always a distinct head before. We call these the GAsTEROPODA. A fourth class is com posed of those where the mouth remains hidden in the bottom of the mantle, which also encloses the branchire and viscera, and is open either througl10ut its length, at both ends, or at one extremity only. Such are our AcEPHALA. A fifth comprises those, which, also inclosed in a mantle and without an apparent head, have fleshy or membranous arms, furnished with cilia of the same nature. We term these BRACHIOPODA. Finally, there are some, which, although similar to the other Mollusca in the mantle, uranchire, &c., differ fmm them in numerous horny and articulated limbs, and in a nervous system more nearly allied to that of the Articulata. · They will constitute our last class, or that of the CIRRHOPODA. (1) M. de Blainville has substituted the name of Malaco:zoaires for that of Molll. llca, separating from them the Ohitons and Cirrhipoda, which he calls .ll'falcnlozoaires. (2) The whole of this arrangement of the Mollusca, .;tnd most of the secondary subdivisions, belong exclusively to me., VoL. 11.-2 0 \ |
