Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/661

Rh MOLLUSC A 633 Classes of the Mollusca. The classes of the Mollusca which we recognize are as follows : Phylum MOLLUSCA. BRANCH A. Glossophora. BRANCH B. Lipocephala ( = Acephala, Cuvicr). Class 1. GASTROPODA. Class 1. LAMELLIBRAXCHIA Br. a. Isopleura. (Syn. Conchifera). Examples Chiton, Neo- Examples Oyster, Mussel, me nia. Clam, Cockle. Br. b. Anisopleura. Examples Limpet, Whelk, Snail, Slug. Class 2. SCAPHOPODA. Example Tooth-shell. Class 3. CEPHALOPODA. Br. a. Pteropodct. Examples Hyalsea, Fneu- modermon. Br. b. Siphonopoda. Examples Nautilus, Cut tles, Poulp. General Characters of the Mollusca. The forms com prised in the above groups, whilst exhibiting an extreme range of variety in shape, as may be seen on comparing an Oyster, a Cuttle-fish, and a Sea-slug such as Doris ; whilst adapted, some to life on dry land, others to the depths of the sea, others to rushing streams ; whilst capable, some of swimming, others of burrowing, crawling, or jump ing, some, on the other hand, fixed and immobile ; some amongst the most formidable of carnivores, others feed ing on vegetable mud, or on the minutest of microscopic organisms yet all agree in possessing in common a very considerable number of structural details which are not possessed in common by any other animals. The structural features which the Mollusca do possess in common with other animals belonging to other great phyla of the animal kingdom are those characteristic of the Ccelomata, one of the two great grades (the other and lower being that of the Ccelentera) into which the higher animals, or Enterozoa as distinguished from the Protozoa, are divided (13). The Enterozoa all commence their indivi dual existence as a single cell or plastid, which multiplies itself by transverse division. Unlike the cells of the Proto zoa, these embryonic cells of the Enterozoa do not remain each like its neighbour and capable of independent life, but pro ceed to arrange themselves in two layers, taking the form of a sac. The cavity of the two-cell-layered sac or Diblas- tula thus formed is the primitive gut or ARCH-ENTERON. In the Ccelentera, whatever subsequent changes of shape the little sac may undergo as it grows up to be Polyp or Jelly-fish, the original arch-enteron remains as the one cavity pervading all regions of the body. In the Coelomata the arch-enteron becomes in the course of development divided into two totally distinct cavities shut off from one another an axial cavity, the MET-ENTERON, which retains the function of a digestive gut ; and a peri-axial cavity, the COSLOM or body-cavity, which is essentially the blood- space, and receives the nutritive products of digestion and the waste products of tissue-change by osmosis. The Mollusca agree in being Coelomate with the phyla Verte- brata, Platyhelmia (Flat-worms), Echinoderma, Appendicu- lata (Insects, Ringed- worms, &c.), and others, in fact, with all the Enterozoa except the Sponges, Corals, Polyps, and Medusae. In common with all other Ccelomata, the Mollusca are at one period of life possessed of a PROSTOMIUM or region in front of the mouth, which is the essential portion of the &quot; head,&quot; and is connected with the property of forward locomotion in a definite direction and the steady carriage of the body (as opposed to rotation of the body on its long axis). As a result, the Ccelomata, and with them the Mollusca, present (in the first instance) the general condition of body known as BILATERAL SYMMETRY; the dorsal is differentiated from the ventral surface, whilst a right and a left side similar to, or rather the complements of, one another are permanently established. In common with all other Ccelomata, the Mollusca have the mouth and first part of the alimentary canal which leads into the met-enteron formed by a special invagination of the outer layer of the primitive body-wall, not to be confounded with that which often, but not always, accompanies the ante cedent formation of the arch-enteron ; this invagination is termed the STOMOD^EUM. Similarly, an anal aperture is formed in connexion with a special invagination which meets the hinder part of the met-enteron, and is termed the PROCTOD^UM. In common with many (if not all) Ccelomata, the Mol lusca are provided with at least one pair of tube-like organs, which open each by one end into the ccelom or body cavity, and by the other end to the exterior, usually in the neigh bourhood of the anus. These are the NEPHRIDIA. Like all other Coelomata, the Mollusca are also provided with special groups of cells forming usually paired or median growths upon the walls of the ccelomic cavity, the cells being specially possessed of reproductive power, and dif ferentiated as egg-cells and sperm-cells. These are the GONADS. As in other Coelomata, the cells of the gonads may escape to the exterior in one of two ways either through the nephridia, or, on the other hand, by special apertures. As in all other Ccelomata, the cells, which build up respectively the primary outer layer of the body, the lining layer of the met-enteron, and the lining layer of the ccelom, are multiplied and differentiated in a variety of ways in the course of growth from the early embryonic condition. TISSUES are formed by the adhesion of a num ber of similarly modified cells in definite tracts. As in all Ccelomata, there is a considerable variety of tissues char acterized by, and differentiated in relation to, particular physiological activities of the organism. Not only the Coelomata but also many Ccelentera show, in addition to the EPITHELIA (the name given to tissue which bounds a free surface, whether it be that of the outer body-wall, of the gut, or of a blood-space), also deeper lying tissues, of which the first to appear is MUSCULAR tissue, and the second NERVOUS tissue. The epithelia are active in throwing off their constituent cells (blood-corpuscles from the wall of the ccelom), or in producing secretions (glands of body-wall and of gut), or in forming horny or calcareous plates, spines, and pro cesses, known as CUTICULAR PRODUCTS (shells and bristles of the body- wall, teeth of the tongue, gizzard, &c.). In the Mollusca, as in all other Ccelomata, in correspond ence with the primary bilateral symmetry and in relation to the special mechanical conditions of the prostomium, the nervous tissue which is in Ccelentera, and even in Flat- worms, diffused over the whole body in networks, tends to concentrate in paired lateral tracts, having a special enlargement in the prostomium. The earlier plexiform arrangement is retained in the nervous tissue of the walls of the alimentary canal of many Coelomata, whilst a con centration to form large nerve-masses (GANGLIA), to which numerous afferent and efferent fibres are attached, affects the nervous tissue of the body- wall. In all Ccelomata, including Mollusca, muscular tissue is developed in two chief layers, one subjacent to the deric or outer epithelium (SOMATIC MUSCULATURE), and a second sur rounding the alimentary canal (SPLANCHNIC MUSCULATURE). Thus, primarily, in Coelomata the body has the character of two muscular sacs or tubes, placed one within the other and separated from one another by the ccelomic space. The somatic musculature is the more copious and develops XVI. 80