Members of the mollusk family have no body segmentation. Instead, their bodies are divided into three body regions: the head, visceral mass, and a "foot." The head of a mollusk contains all the sensory organs and the brain. The visceral mass contains the internal organs. Mollusks also have a foot-like part of their body. This "foot" is a tough part of tissue used to propel them along the substrate on which they live. Mollusks usually have shells, though some do not. The shell is secreted by a region of the body known as the mantle. Many mollusks also have a radula. The radula is akin to a tongue. It is rough and used to scrape at and eat food. The mollusks family is divided into seven groups, the most common being Gastropods, Bivalves, Chitons, and Cephalopods.
The majority of Gastropods make their homes in the ocean, though a small number of snails and slugs make their homes on land. Gastropod literally means "stomach foot," and about 70 percent of mollusks fall into this category. Mollusks like snails, limpets and abalones have shells, but mollusks like nudibranch and slugs have no shell.
Bivalves -- clams, cockles, mussels, oysters and scallops -- are most commonly known because of their purpose as food. Bivalves serve as a major food source for humans, gastropods, fish and sea birds. Most burrow into or attach themselves to the substrate on the ocean floor, filter feeding without the use of a radula. Chitons, however, do have a radula covered in teeth so sharp they can scratch glass. Chitons are slow-moving creatures who crawl along ocean rocks to forage for food. They attach themselves tightly to these rocks, allowing their plated shell to protect them.
Cephalopods do not at first seem to resemble mollusks like snails and sea slugs and bivalves. Cephalopods include sea dwelling animals that swim like squid, octopus, cuttlefish and the nautilus. Their bodies resemble the internal structure of other mollusks. However, it should be noted that squid do have a type of internal shell. Cephalopods have the most highly-evolved nervous system of all mollusks. They also have very good eyesight, though their eye evolved down a different path than the mammalian eye with the same result. All Cephalopods have a beak with which they hunt and eat their prey. Like other mollusks, they also have a mantle, a radula and a U-shaped digestive track.