Examine the image of the snail on page 612 of Campbell 3rd ed., page 604 4th ed., page 609 5th ed. Freshwater mussels are abundently distributed and live on the bottom of lakes, rivers, ponds and streams, where they feed on small plant and animal life. The larval stage of the freshwater mussel is parasitic on the gills of fish. The shell of the clam consists of two valves hinged together along the dorsal side. On the anterior-dorsal part of each valve is a swollen region, the umbo .
Concentric lines extend outward from the umbo and represent lines of growth.
Freshwater mussels are extremely long lived with some species living over 80 years. The valves are lined by a membranous structure, the mantle . At the posterior margin the mantle is thickened and comes together to form two openings, the siphons . Water enters the clam through the ventral incurrent siphon, circulates through the mantle cavity and over the gills , and leaves through the dorsal excurrent siphon. A large muscular foot extends down from the visceral mass.