MOLLUSCA. A large group of invertebrate animals con stituting a large subdivision or phylum of the animal kingdom and comprising such forms as land and aquatic snails and slugs, oysters, mussels, cuttlefish and squids. The group is very large, containing over 6o,000 living species, and, judged by its distribu tion, it has been highly successful, as its members have populated the sea, fresh water and the land. They manifest great diversity of habit and structure, and range in size from the giant squids of the Atlantic, which span over soft. in length, to snails less than a millimetre in length in the adult state.
The phylum is divisible into five classes : the Amphineura (chitons or coat of mail shells), the Gastropoda (limpets, whelks, snails and slugs), the Scaphopoda (or tusk shells), the Lamellibranchia (oysters, mus sels and scallops), and the Cephalopoda (squids, cuttlefish and octopods). Certain problematical fossil forms, the Conularida, are sometimes treated as a class of the Mollusca. Their exact zoological position is, however, debatable, and they cannot be adequately discussed here (see GASTRO PODA).
The Gastropoda are the largest class of molluscs, and easily outnumber the rest. The Scaphopoda are a small group of but two families. In the past such animals as Tunicates, Brachiopods and Cirrhipedes were included in this phylum, and a well defined concept of the latter was only formulated towards the end of last century. Thiele and Odhner at the present time hold the view that the Solenogastres or Aplacophora, a subdivision of the Amphineura, are more closely related in certain respects to certain groups of worms than to the Mollusca. This view is not accepted here and the Soleno gastres are treated as molluscs (see AMPHINEURA).
cept the Lamellibranchia the anterior tegu mentary region is modified as a more or less mobile head usually provided with sen sory appendages and sense-organs. This threefold division of the body constitutes the essential plan of molluscan organiza tion. In addition, it should be noted as es pecially characteristic of the phylum that (I) there are very scanty traces of seg mentation ; (2) the coelom is represented by the pericardium and gonad but is much reduced by the extensive vascular system; (3) the alimentary system (except in the Lamellibranchia) is characterized by the peculiar tongue beset with chitinous teeth (radula) (4) the nervous system consists essentially of a circumoesophageal ring composed of an upper or cerebral half and a lower or labial portion, from the point of junction of which arise longitudinal pallial and pedal cords on each side, and (5) the cleavage of the egg is spiral and segmenta tion usually gives rise to trochophore and veliger larval stages.
The Amphineura are exclusively marine, of elongate shape and symmetrical organization, having the mantle beset with spicules.
In the most familiar forms (Chiton) the shell is divided into eight separate plates. The Gastropoda are asymmetrical and character ized by the atrophy or disappearance of the organs of the original left-hand side, and by the spiral winding of the shell and visceral mass. The Scaphopoda are marine; the mantle is fused in the mid ventral line, so as to enclose the visceral mass in a sheath which secretes a tubular shell. The Lamellibranchia are aquatic : the mantle is divided into right and left lobes which secrete a bivalve shell: the head is undeveloped and the gills become highly com plex, subserving nutrition as well as respiration. The Cephalopoda are marine molluscs in which the head and foot are fused. The edges of this mass are produced into appendages; the ventral surface is much abbreviated and locomotion is mainly accom plished by the ejection of jets of water through an organ (the funnel) developed from the foot.