Snails have a large foot for creeping along surfaces, a single coiled shell that encloses the organs, and a head with eyes and tentacles. Like most other gastropods, snails feed by using a specialized rasping organ called a radula. It is a ribbon-like structure covered with small horny teeth called denticles that tear food into pieces that are then collected by lips or a proboscis. New denticles are constantly being produced to replace those worn away at the front. Snails may be herbivorous or carnivorous, predatory or parasitic. Some feed on plankton and deposits of detritus filtered from the water.