Mollusks first appeared in early Cambrian times, 650 million years ago. They are the second largest animal phylum, Mollusca (from Latin mollis, "soft"). The only larger animal phylum is Arthropoda. According to current estimates, there are approximately 49,000 mollusk species. All mollusks have soft, boneless bodies. Most have shells, though some shells are poorly developed and are even absent in some cases. Mollusk shells are like coats of armor. Mollusks form seven classes. Best known are gastropods, including approximately 38,000 species of snails and slugs; bivalves or pelecypods, approximately 8,000 species of clams and scallops; cephalopods, approximately 600 species of squid and octopuses; and polyplacophora, approximately 600 species of chitons. The other classes contain under 1 percent of mollusk species and are not discussed. Mollusks are present in all of earth's habitats. However, most of them, comprising the greatest species diversity, are in the oceans. There, the largest number are various species of gastropods (snails). All gastropods are univalves, possessing one shell. Many other mollusks are the familiar pelecypod-bivalve-mollusks (clams), which have two shells. Mollusk bodies look undeveloped and many lack apparent heads. However, all have well-developed nervous, circulatory, respiratory, and sensory systems. Except for cephalopods, mollusks are slow-moving, sluggish, or immobile (sessile). In many cases they spend their adult lives attached to rocks or dug into sand or mud, awaiting the approach of prey. Some mollusks, mostly gastropods, occur in freshwater and on land. For example, snails and slugs are seen at the bottoms of ponds, rivers, and lakes, or under fallen trees and decaying logs. Some land snails even live in tree branches. Characteristic features of gastropods are a true head, a creeping surface (or foot), eyes, and tactile feelers. The foot gives gastropods their name, which means "foot on belly" in Latin. Most gastropods have a univalve shell, but some species have no shell. Others, such as garden slugs, have tiny, internal shells.
Thanks for description - Animal life club