Clams use their modified foot to dig and burrow while in octopuses and squids the foot is highly modified into eight tentacles which assist in swimming and grabbing prey. While to the naked eye it looks as if snails use their foot to crawl across the substrate, many actually glide across the sea floor on a slippery secretion. In its most recognizable form such as those found in snails, nudibranchs and sea hares, the foot looks like a muscular hook used to pull the animal across the sea floor and for digging and attachment. The brain or nerve concentration is located at one end of the body in a recognizable head. All of the major organs of the respiratory, digestive and excretory systems are contained in the visceral mass.

All mollusks have a brain or concentrated bundle of nerves, and all have an organ known as a mantle which surrounds the body and is responsible in many species for secreting and maintaining a hard shell.ДЄlthough the many species of mollusks look quite different at first, the bodies of all species are separated into the visceral mass, head and foot. The bodies of mollusks display bilateral symmetry, meaning they are organized front and back. These classes are (1) Gastropoda (the gastropods) which includes all nudibranchs, sea slugs, sea hares, snails, limpets and more (2) Bivalvia (the bivalves) which includes clams, oysters, mussels and scallops (3) Cephalopoda (the cephalopods) which includes octopuses, squids and nautiluses, and (4) Amphineura which includes the chitons. Of these, only members of four classes are commonly witnessed by sport divers and beachgoers. The number of classes in the phylum is six or seven depending on the school of thought of the evolutionary specialist. Of the known species, more than 75,000 occur in saltwater or freshwater environs, and as divers we see one or more species almost every time we dive. Scientists have described more than 110,000 species of mollusks and other species are constantly being discovered. In the animal kingdom, only the Arthropoda, a phylum which includes lobsters, crabs, shrimps and insects, and possibly the phylum which describes worms known as nematodes (not particularly relevant to divers) contains more species.

Species of mollusks are widespread, inhabiting marine, freshwater and terrestrial ecosystems. They possess well-defined organ systems and have sophisticated sensory abilities. In comparison with other invertebrates, mollusks are highly developed creatures. Mollusks, a collection of invertebrates that includes nudibranchs, slugs, snails, chitons, clams, oysters, scallops, mussels, cuttlefishes, octopuses, squids and many other familiar creatures, are a successful group of animals. And despite some obvious differences, all creatures described in the phylum Mollusca are closely related. Giant squid inhabit the waters of the deep ocean while chitons, limpets, sea hares and many mussels inhabit tidal zones. Did you know that the hundreds of species of rainbow colored nudibranchs we so greatly admire as divers are closely related to snails, even those that are considered annoying garden pests? Did you know that many specialists consider octopuses, squids, cuttlefishes and nautiluses the most intelligent of the invertebrates? Many snails are smaller than the fingernail of your little finger while giant squid can grow up to 68 feet (21m) long.

Snails and squids belong to the phylum