To stay alive, they must keep their soft bodies moist at all times and for some like those which live in hot dry deserts environments, this is done by curling up in their shell, secreting a mucous plug and staying holed up until the next bit of moisture comes along. The shell serves both protective, and supportive purposes. The one feature common to all molluscs is the presence of a fleshy mantle . Members of all classes except the bivalves possess a ribbon-like set of hooked teeth called radula . Most molluscs have a well- defined nervous system with a primitive brain.
Molluscs have a circulatory system and most have a two-chambered heart.
Their digestive system usually includes a jaw, pharynx, esophagus, stomach, intestine and anus. Most gastropods and cephalopods have eyes and tentacles. The animal is bilaterally symmetrical. With a well-developed foot surrounded by a groove in which there are 6 to 88 pairs of gills. All chitons are marine inhabitants and most make a living by grazing algae from rocks and other hard substrates.