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by Dr. Gary Rosenberg

Molluscan Classes
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 giant squid.

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.

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