Snail-like animals that naturally lack a shell, or have only an internal shell, are usually called slugs, and land snails that have only a very small shell (that they cannot retract into) are often called semislugs.
Snails that respire using a lung belong to the group Pulmonata, while those with gills form a polyphyletic group; in other words, snails with gills form a number of taxonomic groups that are not necessarily more closely related to each other than they are related to some other groups. Both snails that have lungs and snails that have gills have diversified so widely over geological time that a few species with gills can be found on land and numerous species with lungs can be found in freshwater. Even a few marine species have lungs.
Snails can be found in a very wide range of environments, including ditches, deserts, and the abyssal depths of the sea. Although land snails may be more familiar to people, marine snails constitute the majority of snail species, and have much greater diversity and a greater biomass. Numerous kinds of snail can also be found in fresh water. Most snails have thousands of microscopic tooth-like structures located on a ribbon-like tongue called a radula. The radula works like a file, ripping food into small pieces. Many snails are herbivorous, eating plants or rasping algae from