Bivalve Anatomy

The valve structure and soft anatomy of the bivalves varies among species. The Hard-Shelled Clam or Cherrystone, Mercenaria mercenaria, is used here as an example of a typical bivalve.

Important features of bivalve shells are:

two shelly valves
two adductor muscles (reduced to one in some species) that hold the shell tightly closed when they contract
an elastic ligament that springs the shell open when the muscles relax
a hinge with a series of interlocking teeth that keep the valves aligned
a pallial line that represents the attachment of the soft tissues to the interior of the shell. The presence of a pallial sinus indicates that the species has siphons (that retract into the sinus when the shell closes)

More detailed information about bivalve shells can be found here.

Valve structure of the Hard-Shelled Clam, Mercenaria mercenaria. (Original artwork by Lisa Kanellos, Field Museum of Natural History, published in Seashells of Southern Florida: Bivalves, 2007, courtesy of Princeton University Press)

The shell of the Hard-Shelled Clam (Mercenaria mercenaria), with the muscle scars, pallial line, and pallial sinus outlined on an inside view (upper right). A natural pearl from this species is at lower left. (Photographs by Rüdiger Bieler, published in Seashells of Southern Florida: Bivalves, 2007, courtesy of Princeton University Press)

In life, the shells are lined by a specialized tissue called the mantleMantle:
The external fold, or folds, of the soft, exterior membrane of the body of a bivalve, that secretes the shell and usually forms a cavity enclosing the gills and other organs., which produces and enlarges the shell by secreting new shell material at the free edge. The soft body inside of the shell includes a muscular foot, gills (or ctenidia, used for respiration and feeding), muscles, a digestive system, nerves, a three-chambered heart and an open circulatory system (with sinuses). Some species, such as the Hard-Shelled Clam here, have siphons for directing water flow in and out of the body chamber. More detailed information about bivalve soft anatomy can be found here.

Soft anatomy of the Hard-Shelled Clam, Mercenaria mercenaria. (Original artwork by Lisa Kanellos, Field Museum of Natural History, published in Seashells of Southern Florida: Bivalves, 2007, courtesy of Princeton University Press)

Many species of animals have lost body parts (vestigializationVestigialization:
The evolutionary process of losing nonfunctional traits, usually expressed by a reduction in size or function.) during the course of their evolution. One of the main traits of all bivalves is the absence of a head and its associated organs such as eyes, head tentacles, and mouthparts. Scientists believe that early ancestors of bivalves had heads, like their close relatives the snails, but that bivalves have lost the feature.