Although, at first glance similar to bivalve clams, brachiopods are radically different from the phylum Mollusca to which the clams belong and bear no genetic relationship to them.  Brachiopods have existed since Cambrian time and the phylum, although much reduced, survives to this day.  The bulk of brachiopod species existed during Paleozoic time and they are important index fossils for this period of time.

Brachiopods are organisms attached to the bottom (sessile) by a pedicle and are bilaterally symmetrical.  In contrast, clams tend to be symmetrical along the hinge line and are only rarely bilaterally symmetrical.  There are two classes of brachiopods depending on whether the shell is articulated (hinged) or not.

Class INARTICULATA

The two shells are not hinged and are held together by muscles.  Although geologically unimportant, the genus Lingula holds the survival record for an animal, having existed for over 500 million years since Cambrian time.

Class ARTICULATA

The two shells are hinged and generally more ornate.  They make up the bulk of fossil brachiopod species.

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