by Dr. Gary Rosenberg

What is a Mollusk?

The phylum Mollusca includes organisms such as clams, snails, slugs, octopuses, squid, and chitons. As the phylum is currently defined, several features are common to all or most mollusks:

  • mantle that secretes calcium carbonate in the form of spicules or shell
  • mantle cavity where respiration occurs, usually through the ctenidium (gill) in aquatic forms, or through the mantle wall in air in terrestrial ones, and where excretory and reproductive organs discharge
  • A body divided into three regions, the headfoot, and visceral mass
  • Three coelomic spaces, for kidney, heart and gonad,
  • radula, a ribbon of teeth used in feeding.

The word "mollusk" derives from Latin mollis meaning "soft," just as the term "malacology," the study of mollusks, comes from the Greek word for soft, malakos. Originally Mollusca was used for naked, soft-bodied animals, whereas shelled animals were placed in the Testacea. In the early 1800s, Baron Georges Cuvier realized that gastropods, bivalves, scaphopods, and cephalopods belonged in one group, but he also included barnacles and brachiopods in the Mollusca, which have since been removed. The modern term "shellfish" refers to shelled mollusks and to crustaceans, which are members of the phylum Arthropoda. The term "conchology" is also used for the study of mollusks, although it is sometimes applied to the study of shells alone.

The above material has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells, published by Robert Halt, Ltd., London, 1992.