The bivalves are the second largest group of molluscs with about 8,000 described species. These organisms, as the name implies, have two shells. Most bivalves have two equal sized shells however, some (ex. oysters and scallops), have one shell that is cemented to the substrate, and this gives them an irregular appearance. The foot on these animals is adapted for digging ( as in burrowing clams) or to attach byssal threads (as in mussels). This thread is secreted by the animals, has the tensile strength of steel, and helps to anchor the mussel firmly to the substrate. The radula is absent as most bivalves feed by filtering the water, so their food does not require much grinding up. The head is greatly reduced and they have the unique gastronominal phenomena of having their intestine run through their heart!! Most have primitive "eye spots" that can detect light and dark, and some species when hassled can move by squirting water out of their body cavity. When you introduce your new bivalve to the aquarium, make sure that you place it properly, as they do not like to be moved. 

Here are a few more hints on care and feeding of certain bivalves. 

Bivalves can be divided into two groups. Those that basically feed themselves, and those that require frequent feedings of liquid invertebrate foods and live phytoplankton (floating microscopic plants). Tridacna clams (Giant Clams) actually feed themselves by harvesting algae that is living in their tissues. They require a clean aquarium with powerful lighting, stable temperature, and an occasional supplement of trace elements to thrive. Tridacna clams should be placed in an area of high light and low flow. Place them into the aquarium so that the open part of the shell is oriented up. In a short time the mantle will expand, and insure that there is a good 1 to 2 inches on either side of the animal to permit full mantle expansion.