Phylum Molluscs

A mollusc’s body is covered with a mantle which forms the shell; between the mantle and the body is the mantle cavity. The body cavity is filled with spongy tissue, the heart is well developed and the circulatory system is open. The phylum splits into the classes gastropods, bivalves and cephalopods. The pond snail’s body sits in a spirally coiled shell; the head has a pair of tentacles with eyes at their bases, and it glides by wave-like contractions of the foot muscles. With a toothed radula in its pharynx it scrapes off bacteria and tiny algae; salivary glands and the liver produce digestive juices. It breathes air oxygen with a lung, so it rises to the water surface. The heart has an atrium and a ventricle, and the blood vessels have open ends. There is one kidney and several pairs of nerve ganglia. It is a hermaphrodite and lays eggs in a slimy string. The small pond snail is the intermediate host of the liver fluke; grape snails and slugs harm crops.