Clams, such as the hard clam/northern quahog (Mercenaria mercenaria), are bivalves, meaning that they have shells consisting of two halves, or valves. The valves are joined by a hinge ligament and the adductor muscles hold the shell closed. The shells are designed such that if the adductor muscles are relaxed the shell opens, this is why dead shells are often found in the open position.
Clam Anatomy
The clam's foot is used to dig into sand/sediment and pull itself along the surface of the bottom. A pair of long siphons that extrude from the clam's mantle out the side of the shell reach to the water above (only the exit points for the siphons are shown). Clam's such as the hard clam, are filter feeders; water and food particles are drawn in through the inhalant siphon to the gills where tiny, hair-like cilia move the water, and the food is caught in the mucus on the gills. From there, the food-mucus mixture is transported along a groove to the palps which push it into the clam's mouth. The second siphon carries away the water and waste. The gills also remove oxygen from the water flow.

The mantle, a thin membrane surrounding the body of the clam, secretes the shell. The oldest part of the clam shell is the umbo, and it is from this hinge area that the clam extends as it grows. Clams are protandric hermaphrodes, so as they grow they will change from males (producing sperm) to female (producing eggs).