One of the two main classifications of shellfish (the other being crustacean), mollusks are invertebrates with soft bodies covered by a shell of one or more pieces. Mollusks are further divided into gastropods (also called univalves), such as the abalone and snail; bivalves, like the clam and oyster; and cephalopods, such as the octopus and squid. See also conch; cuttlefish; mussel; periwinkle; scallop; whelk.