by Dr. Gary Rosenberg
The living mollusks are divided into seven classes, APLACOPHORA,
MONOPLACOPHORA, POLYPLACOPHORA, GASTROPODA,
BIVALVIA, SCAPHOPODA, and CEPHALOPODA.
Members of the first two groups are rarely seen outside of museum
collections, the aplacophorans because they are shell-less and
wormlike, monoplacophorans because they live in great depths.
contains about 250 species of marine, wormlike, bilaterally
symmetrical animals. They have no shell, but have calcareous
spicules in the body surface. The foot is restricted to an anterior
pedal shield or to a narrow groove running the length of the
body. Aplacophorans have a radula and a posterior mantle cavity.
Some are detritus feeders, others are predators. They range
in length from 0.04 to 12 inches (1 to 300 millimeters).
is represented by about a dozen living species, the first of
which was discovered in the 1950s. Previously the group had
been known only as fossils dating from 50 to 370 million years
ago. Monoplacophorans have a capped-shaped, limpetlike shell,
a well-developed foot and a posterior anus; they are usually
bilaterally symmetrical. They were once thought to represent
a link to the annelid worms, in having several pairs of foot
retractor muscles, gills, and hearts, but unlike annelids, their
bodies are not truly segmented. Monoplacophorans live at depths
of 650 to 20,000 feet (200 to 6,000 meters); they graze on algae
and microorganisms on hard bottoms. Size ranges from 0.08 to
1.4 inches (2 to 35 millimeters).
Members of the this class, known as chitons, have a shell of
eight usually overlapping plates, held together by a leathery
girdle. The animal is bilaterally symmetrical, with a well-developed
foot surrounded by a groove in which there are 6 to 88 pairs
of gills. The head lacks eyes and tentacles; some species have
light sensing organs (esthetes) in the shell. The radular teeth
contain the mineral magnetite, which hardens them. There are
about 800 species of chitons worldwide. All are marine and many
are grazers on rocks in shallow water, but a few occur in depths
beyond 16,000 feet (5,000 meters). They range in length from
0.1 to 16 inches (3 to 400 millimeters).
also known as tusk shells, are bilaterally symmetrical and have
elongate, tubular, tapering shells that are open at both ends.
The head is rudimentary, and lacks eyes and tentacles. It has
contractile filaments called captacula used in feeding. The
ctenidium is absent and the heart is reduced. The conical foot
can be protruded for use in burrowing. All 350 or so species
are marine; they live buried in muddy or sandy bottoms. Length
ranges from 0.1 to 6 inches (2 to 150 millimeters).
They include the octopus, squid, cuttlefish, and nautilus. They
are bilaterally symmetrical, and often highly streamlined. The
head is surrounded by tentacles and a funnel from the mantle
cavity provides jet propulsion. Members of only a few of the
several dozens of families produce a calcareous shell. Cephalopods
have a concentrated nervous system and include the most intelligent
animals among the invertebrates. All 600 to 650 species are
marine predators or scavengers and have parrotlike beaks or
jaws in addition to the radula. Length ranges from 0.4 inches
(10 millimeters) for male argonauts to 65 feet (20 meters) for
have a single-valved shell which is usually spirally coiled
and is reduced or absent in slugs and semi-slugs. They have
a prominent head with cephalic tentacles and usually a well-developed
foot used in crawling. In the early larval stage of gastropods,
the visceral mass and mantle cavity rotate up to 180 degrees
counterclockwise, in a process called torsion. This brings these
organs from a posterior position to an anterior position behind
the head. The animal in most cases is able to retract into the
shell. An operculum is present in the larvae of most groups,
and is often present in the adults; it serves to seal the aperture
of the shell when the animal is retracted. Gastropods are one
of the few groups of organisms well-represented in marine, freshwater,
and terrestrial habitats. There are about 60,000 species. Size
ranges from 0.02 to 30 inches (0.5 millimeters to 0.75 meters).
have two valves connected by a flexible ligament. The mantle
cavity is enlarged, enclosing the visceral mass and other internal
organs. There is no differentiated head or cephalic region and
the radula is absent. Most bivalves are filter feeders, with
the gill acting as a food collecting and sorting organ, in addition
to filling its respiratory function. The mouth usually has a
pair of labial palps on either side, which handle food collected
by the gill. There are about 10,000 living species of bivalves
in marine and freshwater habitats worldwide. Size ranges from
0.02 to 50 inches (0.5 millimeters to 1.3 meters).
When identifying bivalves,
you must be able to distinguish anterior from posterior and
left valve from right valve. The left valve in members of one
family might look like the right valve in another. In general,
if the ligament is restricted to one side of the beak, it is
on the posterior side. Also, if the shell has a pallial sinus,
it is posterior. If you hold a bivalve beaks upward, with one
valve in each hand and its posterior end toward you, the right
valve is in your right hand, the left in your left.
The above material
has been adapted from Dr. Rosenberg's The Encyclopedia of Seashells,
published by Robert Halt, Ltd., London, 1992.