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:
- A mantle that
secretes calcium carbonate in the form of spicules or shell
- A 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
- A body divided into three
regions, the head, foot, and visceral
- Three coelomic spaces,
for kidney, heart and gonad,
- A 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.