The Seven Classes of Mollusks

by Richard L. Goldberg

· Gastropod (Univalve)

· Pelecypod (Bivalve)

· Polyplacophora (Chiton)

· Scaphopod (Tusk)

· Cephalopod (Nautilus)

· Aplacophora

· Monoplacophora

The Gastropods are the largest class of mollusks, with over 60,000 species known to science.
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   Fig. 2 / Bivalves are clams. Are two shells better than one?
BIVALVES: This class of mollusks is better known as clam shells. The appropriately named bivalve ("bi" meaning two) consists of two shell halves attached by a flexible corneous hinge. The class Bivalvia, or the Pelecypods are the second most populous class of mollusks.
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   Fig. 3 / Polyplacophora - Tonicia schrammi hitching a ride on Chiton tuberculatus are species from two different Families of Chitons.
POLYPLACOPHORA: The Polyplacophora (polly - plack - afora) or Chitons (kitons) are a primative form of mollusk with eight shelly plates held together by a fleshy girdle.
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   Fig. 4 / Tusk Shells - Like miniature elephant tusks, the Scaphopods are found buried in sand with the tapered end sticking up through the surface.
SCAPHOPODS: Scaphopods are one of the most curious of mollusks. Most look like a miniature elephants tusk, and in fact, one species is known as the Elephant's Tusk, Dentalium elephantium. Some Scaphopods grow to be as large as a human hand or as small as a grain of rice.
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   Fig. 5 / The Nautilus - The Cephalopods include the squid and octopus. Yes, an octopus is a mollusk, but not all Cephalopods form a Nautilus-like shell.
CEPHALOPODS: The most recognizable Cephalopod shells are the Nautiluses. Two flavors of Nautilus from two different subclasses of Cephalopods are known -- the true Nautilus, and the Paper Nautilus.
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   Fig. 6 / Aplacophora - One of the least know of the molluscan classes.
APLACOPHORA: The Aplacophora are worm-like mollusks from abysal depths of the ocean.
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   Fig. 7 / The Monoplacophora have a sand dollar-like appearance. This primitive form of mollusk inhabits very deep water.
MONOPLACOPHORA: The Monoplacophora were once thought to be extinct. Until the early 1960's, scientists only knew this primitive form of mollusk from the fossil records. Deep sea research boats dropping trawl nets in hundreds of fathoms of water and began to discover living examples of this primitive form of mollusk.
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