The phylum Molluska is one of the largest of all animal groups. Not only does it contain about 110,000 living species, the f ossil record indicates a long and extensive history.

The major mollusk groups, called classes by most taxonomists (scientists who study the relationships of plants and animals), are the Gastropoda (snails and slugs), the Bivalvia (clams, mussels, and shipworms), the Cephalopoda (squid, octopuses, cuttlefish, and nautilus), the Polyplacophora (chitons), the Scaphopoda (tooth shells), the Monoplacophora (a single-shelled animal), and the little-known Aplacophora (a questionable mollusk).

Of all animal phyla, the mollusks are perhaps the most difficult to describe in terms of a "typical" mollusk. In fact, no one characteristic is unique to the mollusks and shared by all species. Their body shapes are immensely different. In terms of feeding and behavior, mollusks range from docile, grazing herbivores to stealthy and aggressive predators. Most mollusks are marine, except...