One of the two marine groups into which shellfish are categorized. A second group is crustaceans. Mollusks are characterized by having a hard shell that consists of one or two pieces which contains a soft body. The mollusks have two sub-classifications, which are univalves and bivalves. The univalves have one shell, which include shellfish such as abalone, conch, snails, and squid. The bivalves have two half shells, which include shellfish such as clams, mussels, oysters and scallops.