Shellfish may be divided into four distinct classifications.
Crustacean -These are animals with a body, outer jointed skeleton or shell. This group includes lobster, crayfish, crab, shrimp, and prawn, among others. This class sheds its shell periodically as it grows larger.
Mollusks -These are animals which are univalve (one shell) or bivalve (two shells). Then shells expand as the animal grows. Snails, Conch, and Abalone are univalve. Clams Oysters, and mussels are bivalve.
Cephalopods -Though technically classed as mollusks, these animals have a reduced internal shell called a pen or cuttlebone, as in the case of the cuttlefish. This class includes squid, octopus and cuttlefish. These fish share many of the same preparation methods with the crustacean and mollusk classes.
Echinoderms - Symmetrical animals that are only found in the sea (there are none on land or in fresh water). Echinoderms mean "spiny skin" in Greek. Many, but not all, echinoderms have spiny skin. There are over 6,000 species.
Most adult echinoderms live on the bottom of the ocean floor. Many echinoderms have suckers on the ends of their feet that are used to capture and hold prey, and to hold onto rocks in a swift current