BIVALVES
Members of the class Bivalvia (a.k.a. Pelecypoda) are so called because of their shell, which is composed of two valves. The first photograph shows the inside of a valve of Mercenaria campechiensis (Carter Coll. RGa874; St. Joseph Spit, Florida) showing some of the important internal parts. Various marks on the inside show the form of the soft parts, notably the two adductor muscle scars that mark the attachment of the muscles responsible for pulling the shell closed. The pallial line marks the attached edge of the mantle, a sheet of tissue that enclosed the other soft parts in a mantle cavity and secreted the shell. The pallial sinus marks the region where the mantle was modified and folded outward to make a tubular structure called the siphon, which pulled water into/flushed water out of the mantle cavity. These currents brought food and oxygen-rich water to the animal and flushed waste matter and gasses out. Species with a sinus needed these siphons because they burrowed into the sediment. The dentition was a set of projecting plates (teeth) on the hinge and matching sockets into which they fit on the opposite valve. These assured correct alignment of the valves as the shell closed and made it difficult to open the shell by twisting the valves in different directions. The beak is simply that part of the external shell surface that extends above and (in some cases) toward the hinge. Between the dentition and beak a deep groove is apparent. This is the location of the ligament, a tough, elastic organic structure that (in this species) stretched when the shell was pulled closed and contracted when the adductor muscles relaxed, thus opening the shell.
The photo at right above is a rubber cast of a clam called Panopea generosa, the Geoduck (pronounced "gooeyduck") of the American west coast. Note the size: roughly 36cm (15") long. The shell is at bottom and the elongate structure poking out of it at the top is the siphon. the Geoduck is the official mascot of Evergreen State University in Olympia, WA (where I bought this example). Students and fans take these rubber casts to sporting events, etc. and wave them around in the air.
The photograph below at left is of Tapes phillipinarum, also from western North America (Carter Coll., RGa269). This one was collected from the trash pile of a clam bake in Olympia, WA. In this view, along the hinge and toward the anterior end, the symmetry of the shell is evident. Also shown are annulae (or growth rings -- parallel to the edge of the shell) and costae (or ribs -- perpendicular to the annulae). Note that the umbo is the region of the shell outside the hinge and near the beaks -- the early growth stages of the shell, in other words.
The photograph to the right below shows the umbonal region of Chione intapurpurea (Carter Coll. RGa64) from Jacksonville, FL. It shows the dentition from a different angle, making obvious the distinction between the cardinal and lateral teeth. Also, just in front of the beaks is a flattened area called the Lunule that aids in burrowing correctly. Behind the beaks lies the escutcheon, a depression around the elongate ligament. The hole was bored by a predatory snail, probably Polinices duplicatus which is shown on one of the gastropod pages.
Bivalves like the three shown above, with asymmetric, aragonite valves that are mirror images of each other are informally called "clams". Two other major groups of bivalves differ in being entirely asymmetric (or practically so) and in having shells primarily made of calcite. These are the "oysters" (left photo below) and the scallops (right photo below). The oyster (Crassostrea virginica Carter Coll, RG1329 from Jacksonville, FL) has a shape typical of those living in the open ocean: the larger valve (shown) is roughly triangular and elongate and the smaller, flat valve fits pretty closely into it. In crowded populations in marshes and estuaries the shell form becomes even more irregular. At first glance the scallop valve looks symmetric, but close inspection the anterior end is a bit fatter and the wing more irregularly shaped than the posterior. In some cases there is a distinct notch (the byssal notch) between the anterior ala and the main part of the shell that does not occur on the posterior ala. In some species each valves is roughly equally inflated, giving the shell a typical bivalve symmetry, in others one valve is less inflated or even flat or slightly concave.
