Any member of the class Gastropoda, the largest group of mollusks, including about 65,000 species. Gastropods, which include the snails, conchs, whelks, limpets, periwinkles, abalones, slugs, and sea slugs (see nudibranch), are found worldwide, in marine, freshwater, and terrestrial environments. Gastropods typically have a large foot with a flat sole for crawling, a single coiled shell that covers the soft body, and a head that bears a pair of eyes and tentacles. However, they are so diverse that some forms lack shells, while animals in one genus have shells with two halves, like bivalves. Most feed by using a radula, a ribbon of small horny teeth that tear food into pieces. They may be herbivores, carnivores, predators, parasites, or filter feeders of plankton and detritus.