Bivalves: Bivalvia

Bivalves have bodies that are flattened from side to side and completely surrounded by two shells called valves. Each valve is made up of a hard mineral called calcium carbonate and is joined to the other by a hinge on the back. Interlocking teeth or sockets form the hinge on the valves, which are then held together by an elastic ligament or fiberlike tissue made up mostly of protein. Powerful muscles contract ligaments to keep the valves closed and relax to open them. The bulge near the hinge is called the umbo and is the oldest part of the shell. The ridges that form around the umbo trace the growth of the valve. The valves are usually similar to one another in size, shape, color, and texture. The outer surface is usually plain, but some species have distinctive colors and patterns.

Inside the valves is the body surrounded by the mantle. The mantle makes the calcium carbonate that forms the valves. In some bivalves, the rear edge of the mantle forms special tubelike openings called siphons (SAI-fens), which take in water carrying bits of food and expel waste, eggs, and sperm into the water. The mantle holds digestive and reproductive organs and a muscular foot. The inside lining of the stomach is tough enough to grind up food. The foot can stretch outward and attach the bivalve to rocks, wood, and other hard surfaces. The very small head lacks the eyes, tentacles, and radula found in most other mollusks. The radula (RAE-jeh-leh) is part of the mouth that is thick or ribbonlike and has rows of teeth. Light-sensitive organs called eye spots may be found on other parts of the body.