This image shows a number of anatomical features of a preserved, commercially prepared dissection mount of a freshwater mussel. Note the large, hatchet-shaped foot (1) that is used for burrowing into the substrate.  The heart (5) has been injected with red latex, the gills (4) with blue latex and the intestine (9) with yellow latex.  Digestive wastes are discharged from the intestine into the mantle cavity through the anus (10). Bivalves are distinguished from other molluscs by being having laterally compressed bodies encased in two shells (valves) that are held together by a dorsal hinge ligament (6) that causes the valves to open ventrally. The valves are drawn together by a pair of anterior (2) and posterior (8) adductor muscles, which are the parts of edible scallops that are eaten.  In terms of nutrition, most bivalves are sedentary filter feeders. The posterior edges of the mantle (7) are modified to form a ventral incurrent siphon (11) that brings food and oxygen into the animal and a dorsal excurrent siphon (12) that takes carbon dioxide and wastes out.