Snails and slugs are gastropods, which make up the largest class of mollusks with more than 60,000 species. Most of these species can be identified by their shells. Some dwell in ocean, others in the freshwater of rivers, ponds, and lakes. Land snails abound in tropical jungles and in damp temperate regions. All of them need calcium carbonate for building their shells, and so are not common in sandy soil.
Slugs differ from snails in that they generally have only a small internal shell.
Snails move by sliding on their single foot. Specialized gland in the foot secrete mucus, which lubricates the path over which the snail crawls. Snails can only crawl. Even those that live in water can't swim. As they crawl they secrete a slime to help themselves move across surfaces.