mollusk

Any invertebrate of the phylum Mollusca with 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 mollusks have arms to capture their prey. The majority of mollusks 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.

Mollusks vary in diet, the carnivorous species feeding chiefly upon other members of the phylum. Some are vegetarian. Reproduction is by means of eggs and is sexual; many species are hermaphrodite. The shells of mollusks 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 mollusk has a fold of skin, the mantle, which covers the whole body or the back only, and secretes the chalky substance that forms the shell. The lower ventral surface (belly area) of the body forms the foot, which enables the mollusk to move about.