There are four major groups of mollusks: 1) chitons; 2) gastropods, which include snails, slugs, and nudibranches; 3) bivalves, which include clams, oysters, and mussels; and 4) cephalopods, which include squids and octopods. Although mollusks vary widely in external appearance, most share the following body plan: 1) a muscular foot, usually used for movement; 2) a visceral mass containing most of the internal organs; and 3) a mantle—a fold of tissue that drapes over the visceral mass and secretes a shell (in organisms that have a shell).