Mollusks type Mollusca;

A very numerous type of invertebrate, second only to arthropods in numbers. Most mollusks live in water and most of these are marine. The best known classes of mollusks are snails (class Gastropoda), clams (class Bivalvia) and octopuses and squids (class Cephalopoda). Mollusks usually have a bilaterally symmetrical body. However, internal organs in some mollsuks can shift, creating asymmetry. Mollusks body is composed of a head, a thorax and a foot or arms serving, among others, as a means of locomotion. Throax is surrounded by mantle which can produce skeleton (by most mollusks commonly called shell).

Mollusks have the following systems: nervous, digestive, respiratory (gills in aquatic an lungs in land mollusks), reproductive, excretory, and circulatory. Mollusks reproduce sexually and the sexes are mostly separate.