Phylum Mollusca

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The molluscs rival the arthropods in their diversity of body forms and sizes, as well as their ecological success. The phylum also provides some of the most familiar animals, such as snails , clams , mussels , squids , and octopus (which, like the arthropods , are well known because they're good to eat). The phylum Mollusca also includes lesser known forms such as the chitons , tusk shells, solenogasters , among others. Approximately 50,000 species of Molluscs have been described, and because of the shelled forms they have left a rich fossil record. However, the earliest molluscs probably arose in the Precambrian , and nothing is known about what they were like.

Systematic summary for the phylum Mollusca :
Class Gastropoda ( snails, sea slugs)
Class Pelecypoda (=Bivalvia: clams, mussels, oysters, scallops)
Class Cephalopoda (squids, octopus, nautilus, cuttlefish)
Class Aplacophora
Class Monoplacophora
Class Polyplacophora (chitons)
Class Scaphopoda (tusk shells)
Class Caudofoveata

CLASSES OF MOLLUSCS
Class Gastropoda



Hermissenda crassicornis , Nudibranch (sea slug),class Gastropoda , Order Nudibranchia , from McFarland, 1966.
Most of the approximately 40,000 living species of gastropods have shells, however there are quite a few groups that have either reduced or internal shells, or no shell at all. Shelled forms are generally called "snails " and forms without shells are called "slugs", however the terrestrial slugs are not closely related to the various marine forms that are without shells. Although most Gastropods are marine, there are numerous forms in both freshwater and terrestrial environments.
Class Pelecypoda (=Bivalvia )


Scallop , Pecten sp., class Pelecypoda , Pratt, 1923.
The bivalves include forms such as clams , mussels , scallops , oysters , as well as many less familiar forms. Bivalves are laterally compressed animals, with two shell "valves " that are hinged on the an...