mollusc

Any of a group of invertebrate animals, most of which have a body divided into three parts: a head, a central mass containing the main organs, and a foot for movement; the more sophisticated octopuses and related molluscs have arms to capture their prey. The majority of molluscs are marine animals, but some live in fresh water, and a few live on land. They include clams, mussels, and oysters (bivalves), snails and slugs (gastropods), and cuttlefish, squids, and octopuses (cephalopods). The body is soft, without limbs (except for the cephalopods), and cold-blooded. There is no internal skeleton, but many species have a hard shell covering the body. (Phylum Mollusca.)

Molluscs have varying diets, the carnivorous species feeding mainly on other molluscs. Some are vegetarian. Reproduction is by means of eggs and is sexual; many species are hermaphrodite (having both male and female reproductive organs). The shells of molluscs take a variety of forms: single or univalve (like the snail), double or bivalve (like the clam), chambered (like the nautilus), and many other variations. In some cases (for example, cuttlefish and squid), the shell is internal. Every mollusc has a fold of skin, the mantle, which covers either the whole body or only the back, and secretes the chalky substance that forms the shell. The lower ventral surface (belly area) of the body forms the foot, which enables the mollusc to move about.