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mollusc
A member of the phylum Mollusca, which includes snails, bivalves, squids, and octopuses. Molluscs are invertebrates that have fleshy, muscular bodies and typically a chalky shell for protection. There are over 80,000 living species, and these are classified into three main classes: Gastropoda, Bivalvia, and Cephalopoda.
Gastropods, which include slugs, which lack shells, and snails, which possess shells, have bodies divided into a head, a visceral hump, and a muscular foot. The head bears tentacles. During development, the visceral hump twists counterclockwise through 180°, a process known as torsion.
Bivalves, which include oysters, mussels, and clams, are enclosed in shells that form a pair of hinged valves that are drawn together by an elastic ligament.
Cephalopods, which include squids, octopuses, and cuttlefish, typically bear tentacles and their shell is enclosed within their bodies. The nervous system is well developed and the eyes are extraordinarily similar to those found in vertebrate animals such amphibians or mammals. Locomotion is by crawling on the sea bed or by jet propulsion using the sudden ejection of water through a siphon.
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