96 



MOLLUSCA 



Classes of the MoUusca. — ^The classes of the Mollusca 

 which we recognize are as follows : — 



Phylum MOLLTTSOA. 



Branch A. — Glossophora. Branch B. — Lipocephala 



(=Acepliala, Ouvier). 



Class 1. — Gastropoda. Class 1. — Lamellibeanchia 



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



Examples — Chiton, Neo- Examples — Oyster, Mussel, 



menia. Clam, Cockle. 

 Br. b. — Anisoplev/ra. 



Examples — Limpet, Whelk, 

 Snail, Slug. 



Class 2. — SCAPHOPODA. 



Example — Tooth-shell. 



Class 3. — Cephalopoda. 

 Br. a. — Pteropoda. 

 Examples — Hyalaea, Pneu- 

 modennon. 

 Br. b. — Siphonopoda. 

 Examples — Nautilus, Cut- 

 tles, Poiilp. 



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 ; whUst 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 MoUusca do possess 

 in common with other animals belonging to other great 

 phyla of the animal kingdom are those characteristic of 

 the Coelomata, one of the two great grades (the other and 

 lower being that of the Coelentera) 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 

 Uke 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 aech-enteeon. 

 In the Coelentera, 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-eijteeon, 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 Coelomate with the phyla Verte- 

 brata, Platyhehnia (Flat-worms), Echhioderma, Appendicu- 

 lata (Insects, Kinged-worms, &c.), and others, — in fact, 

 with all the Enterozoa except the Sponges, Corals, Polyps, 

 and Medusae. 



In common with all other Coelomata, the MoUusca 

 are at one period of life possessed of a prostomictm 

 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 Coelomata, and with 

 them the Mollusca, present (in the first instance) the general 



condition of body known as bilateeai, symmetry; the 

 dorsal is dififerentiated 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 aU other Coelomata, the MoUusca 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^um. 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 PEOCTOD^UM. 



In common with many (it not aU) Coelomata, the Mol- 

 lusca are provided with at least one pair of tube-Uke organs, 

 which open each by one end into the coelom or body cavity, 

 and by the other end to the exterior, usually in the neigh- 

 bourhood of the anus. These are the nepheedia. 



Like aU 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 ceUs 

 being speciaUy possessed of reproductive power, and dif- 

 ferentiated as egg-cells and sperm-cells. These are the 

 GONADS. As in other Coelomata, the ceUs 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 aU other Coelomata, the cells, which build up 

 respectively the primary outer layer -of the body, the 

 Uning layer of the met-enteron, and the lining layer of the 

 coelom, 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 

 Coelomata, there is a considerable variety of tissues char- 

 acterized by, and dififerentiated in relation to, particular 

 physiological activities of the organism. Not only the 

 Coelomata but also many Coelentera show, in addition to 

 the EPITHELIA (the name given to tissue which bounds a 

 free surface, whether it be that of the outer body-waU, of 

 the gut, or of a blood-space), also deeper lying tissues, 

 of which the first to appear is mtjsculab tissue, and the 

 second neevotts tissue. 



The epithelia are active in throwing ofif their constituent 

 cells (blood-corpuscles from the wall of the coelom), or in 

 producing secretions (glands of body-wall and of gut), or 

 in forming homy or calcareous plates, spines, and pro- 

 cesses, known as cttticulae peoducts (shells and bristles 

 of the body-wall, teeth of the tongue, gizzard, &c.). 



In the MoUusca, as in all other Coelomata, 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 Coelentera, 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 earUer 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-waU. 



In aU Coelomata, 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 aUmentary 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 



