Denizens of both the land and sea, mollusks are so diverse that it is difficult to describe them as a single group. There are mollusks like the clam, which spends its life enclosed in a hinged shell, or the slug, which lacks a shell entirely and is free to move its slimy body across land. Then there is the octopus, with no shell, large eyes, and eight long tentacles, or the nautilus, which lives in a shell but is free to swim with the aid of a few dozen tentacles. Despite their differences, mollusks do share several things in common.
Mollusks have three distinct divisions of their body. The head contains the sensory equipment (eyes, antennae, etc) and the primitive brain. The visceral hump, which is the main body, contains most of the organs, including a complete digestive and excretory tract as well as the reproductive organs. The visceral hump also includes the two external flaps of tissue, which are known as the mantle. The mantle excretes the material that forms the shell on some species, and it protects the mantle cavity. The mantle cavity contains the gills, which excrete waste and circulate oxygen. Most mollusks have a shell, which sits on the visceral hump and acts as a "home", much like a turtle shell. It protects the main body from predators.
The third body section is the foot, which enables locomotion. Even clams have a foot.
Most mollusks have a special tongue-like organ known as the radula. The radula is composed of chitin, much like the exoskeleton of an arthropod, and is covered in pointy "teeth". The radula can be used to scrape food off rocks and to kill larger prey.
Mollusks comprise a large portion of the seafood industry, and are considered delicacies in certain countries. Clams, oysters, scallops, octopuses, muscles, squid, abalone, chitons, and snails are all considered to be "good eats" in North America.
Mollusks are mostly aquatic, being found in all the oceans. Some species prefer freshwater, and can be found in ponds and streams far inland. Some species of snails and slugs are land dwellers, and can be found in most areas that contain damp soil and ample food, such as a garden.
There are roughly 50 000 known species of mollusks (this estimate has been lowered from 150000 species) in 7 classes:
Aplacophora (aplacophorans - lack shells) 250 spp
Bivalvia (clams, oysters, muscles, scallops) 7500 spp
Cephalopoda (octopuses, squid, nautiluses, cuttlefish, argonauts) 600 spp
Gastropoda (snails, slugs) 37 500 spp
Monoplacophora (monoplacophorans - 1 shell) 10 spp
Polyplacophora (chitons) 600 spp
Scaphopoda (tusk shells) 350 spp
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