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Lacking a head and a tongue better known as a radula, bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. They animals have laterally compressed bodies enclosed by a shell consisting of two hinged parts that keep the shells attached. Bivalves have an open circulatory system that bathes the organ in blood. The heart of bivalves has three chambers: two auricles receiving blood from the gills, and a single ventricle, as opposed to humans who have two ventricles . The ventricle is muscular and pumps hemolymph into the aorta, and then to the rest of the body. Oxygen is absorbed into the hemolymph, their blood, in the gills which provide the primary respiratory surface. The hemolymph, blood, lacks an actual pigment with some of the pelecypods having blue while o there have red blood.