96 



MOLLUSCA 



Classes of the Mollusca. The classes of the Mollusca 

 which we recognize are as follows : 

 Phylum MOLLUSCA. 



BRANCH A. Glossophora. BRANCH B. Lipocephala 



( = Acephala, Cuvier). 



Class 1. GASTROPODA. Class 1. LAMELLIBRANCHIA 



Br. a. Isopleura. (Syn. Conchifera). 



Examples Chiton, Neo- Examples Oyster, Mussel, 



menia. Clam, Cockle. 

 Br. b. Anisopleura. 

 Examples Limpet, Whelk, 

 Snail, Slug. 



Class 2. SCAPHOPODA. 

 Example Tooth-shell . 



Class 3. CEPHALOPODA. 



Br. a. Pteropoda. 

 Examples Hyalsea, Pneu- 

 modermon. 



Br. Jt. 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 Ccelomata 

 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 CCELOM 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 Ccelomate 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 " head," 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.EUM. 



In common with many (if not all) Coelomata, 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 Ccelomata, 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 Ccelomata, 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 

 Ccelomata 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 NEEVOUS 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 Ccelomata, 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 Ccelomata 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 



