Bivalves are well-known marine organisms. There are many bivalves that cover the ocean floor. Some types are clams, scallops, quahogs, oysters, mussels, and cockles. In some parts of the sea floor, there are over eight thousand different species living together in an area around 10 square feet.
Bivalves are similar to gastropods, animals like octopuses and squids, because they are both mollusks. The difference between the two is that bivalves' shells are divided completely into two halves, the valves, which open using small hinges. The valves protect and enclose the soft body that's on the inside of the shell.
Also distinguishing them from gastropods, bivalves are less active. Because bivalves are unable to travel very far outside of their shells, many live in sand or mud. Others hide in rock crevices, attach themselves to hard surfaces under water, burrow underground, or live deep under the ocean. They live in both fresh and marine waters.
Most bivalves start out in a larval stage before growing bigger and taking on more characteristics that distinguish them as bivalves. As the animals continue to grow, their shells grow right along with them. Bivalves only open their hinges when they want to move. Their shells are made up of calcium carbonate that come out of the mantle or the soft inside wall of their bodies.
Bivalves are mostly filter feeders, which means that they draw currents of water through their gills, then food particles from the water. They also extract oxygen they need to survive from the water. Those bivalves, like deep-sea scallops, that are not filter feeders will prey on small crustaceans like shrimp or fish.
The phylum of a bivalve is Mollusca and the class is bivalvia. There are over fifteen thousand different species of bivalves, and all have their own sub-class and order.
[Source: Wikipedia, Eyewitness Books: Shell, Encyclopedia.com]