Seashell

A shell is the exoskeleton of a mollusk. The majority of shelled mollusks are found in the class of gastropods such as snails and bivalves such as those in oysters. Three other classes of mollusks secrete a shell systematically: the scaphopods (dental), the Polyplacophora (chitons) and monoplacophorans. It is also the case for almost all cephalopods, although sometimes it is reduced and the mantle ("bones" of cuttlefish or "pen" squid).