What they are
Clams, Mussels, Oysters, Scallops, Shipworms
Morphology
The pelecypods are a type of mollusk that has two shells, hence the common name of bivalves. The shells are hinged together by a ligament that continually forces the shells open. Only special muscles, called adductor muscles, can close the shells together. Upon death, the shell springs open since the adductor muscle fails to work. This is why most fossil bivalves are found open or in two pieces.
The two shells are symmetrical with respect to each other. In other words, the top shell is a mirror image of the bottom shell. This is important to note, since bivalves and brachiopods look very similar. The difference is found in their respective symmetries. Bivalves are symmetrical with respect to their hinge line while brachiopods have a line of symmetry perpendicular to the hinge line, that is, the left of the top and bottom shells is identical to the right of the top and bottom shells.
Like all mollusks, bivalves have a foot. Some species use this foot to crawl along the sea floor. Some use their foot to burrow into the mud of the sea floor; a few can even burrow into rocks or wood (shipworms). Some species don't even use their foot for locomotion, like scallops, which flutter their two shells together rapidly, creating a small jet of water to "swim" through the water.
Ecology
Bivalves live in a variety of environments, from marine to brackish (marine and freshwater interface) to freshwater. They can also be in any climate, ranging from sub-artic to tropical.
Most bivalves live in relatively shallow, nearshore waters (typically less than a few hundred meters). However, there are a few species that make their homes in deep waters, like those of the abyssal plain region of the sea floor.