The phylum mollusca is a large group of animals that includes octopuses, snails, and scallops. Octopuses and their relatives— including squid and cuttlefish—form the class Cephalopoda. Cephalopods typically have a head with a radula (a filelike feeding organ) and beak; a well-developed nervous system; sucker-bearing tentacles; a muscular mantle (part of the body wall) that can expel water through the siphon, enabling movement by jet propulsion; and a small shell or no shell. Snails and their relatives—including slugs, limpets, and abalones—make up the class Gastropoda. Gastropods typically have a coiled external shell, although some, such as slugs, have a small internal shell or no shell; a flat foot; and a head with tentacles and a radula. Scallops and their relatives—including clams, mussels, and oysters—make up the class Bivalvia (also called Pelecypoda). Features of bivalves include a shell with two halves (valves); large gills that are used for
breathing and filter feeding; and no radula.
