MOLLUSK

also spelled Mollusc, any soft-bodied invertebrate animal (phylum Mollusca) usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Mollusks constitute a diverse animal group, comprising about 75,000 species that may be found in most habitats from the deep sea to high mountains. Mollusks are thought to have evolved from a flatwormlike ancestor. Fossils of certain early marine forms date back to the Cambrian period (about 570 million to 505 million years ago). Living mollusks are generally classified into eight major groups: Tryblidia (neopilinids), Gastropoda (limpets, snails, whelks, slugs), Bivalvia (clams, mussels, oysters, scallops), Scaphopoda (tusk shells), Solenogastres (narrow-footed gliders), Cephalopoda (squid, cuttlefish, octopuses, nautiluses), Caudofoveata (mudmoles), and Placophora (chitons). Mollusks are economically important. Many aquatic species, particularly of clams, oysters, scallops, and mussels, are edible and fished or raised commercially. Common land snails of the Helix species are table delicacies in Europe. Various kinds of mollusk shells and pearls from oysters are widely used to make jewelry and other decorative items. On the other hand, several forms, such as shipworm clams, cause millions of dollars worth of damage each year to wharves and wooden-hulled boats. also spelled mollusc, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most numerous groups in the animal kingdom. Each class includes an ecologically and structurally immense variety of forms: the shell-less Caudofoveata; the narrow-footed gliders (Solenogastres); the serially valved chitons (Placophora or Polyplacophora); the cap-shaped neopilinids (Tryblidia); the limpets, snails, and slugs (Gastropoda); the clams, mussels, scallops, oysters, and cockles (Bivalvia); the tubiform to barrel-shaped tusk shells (Scaphopoda); and the nautiluses, cuttlefishes, squids, and octopuses (Cephalopoda). Additional reading An extensive and updated treatment of molluscan structure, function, and evolution is Karl M. Wilbur (ed.), The Mollusca, 12 vol. (198388). The phylum is outlined in Kenneth J. Boss, Mollusca, in Sybil P. Parker (ed.), Synopsis and Classification of Living Organisms, vol. 1 (1982), pp. 9451166; while J.E. Morton, Molluscs, 5th ed. (1979), is a general discussion of their biology. Libbie Henrietta Hyman, The Invertebrates, vol. 6, Mollusca I (1967), contains information on the lower groups and a classic summary of gastropods. Other overviews include Alan Solem, The Shell Makers: Introducing Mollusks (1974); C.M. Yonge and T.E. Thompson, Living Marine Molluscs (1976); and, on three individual groups, Luitfried V. Salvini-Plawen, Schild- und Furchenfsser (Caudofoveata und Solenogastres) (1971); and Piet Kass and Richard A. Van Belle, Monograph of Living Chitons (1985 ).