Within the shell is a fleshy layer of tissue called the mantle; there is a cavity (the mantle cavity) between the mantle and the body wall proper. The mantle secretes the layers of the shell, including the inner nacreous, or pearly, layer. Sometimes a pearl is formed as a reaction to irritation, by the depositing of nacreous layers around a foreign particle. The head is much reduced, without eyes or tentacles, and a muscular hatchet-shaped foot projects from the front end of the animal, between the valves. The foot is used for burrowing, and, in some bivalves (e.g., razor clams), to swim. Many bivalves have two tubes, or siphons, extending from the rear end: one (the incurrent siphon) for the intake of oxygenated water and food and one (the excurrent siphon) for the outflow of waste products. The two tubes may be joined in a single siphon, or "neck."
The gills, suspended within a mantle cavity, are usually very large and function in food gathering (filter feeding) as well as in respiration. As water passes over the gills, tiny organic particles are strained out and are carried to the mouth. Members of the order Septibranchia, however, lack gills and feed on small crustaceans and worms.
Bivalves have a complete digestive tract; a reduced nervous system; a complete, open circulatory system with a chambered heart, arteries, veins, and blood sinuses; and excretory and reproductive organs. In most species the sexes are separate, and the eggs and sperm are shed into the water, where fertilization occurs. The larval stage is free-swimming and lacks a shell.
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