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A snail is a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod . However, the common name snail is also used for most of the members of the molluscan class Gastropoda that have a coiled gastropod shell that is large enough for the animal to retract completely into. When the word "snail" is used in this most general sense, it includes not just land snails but also numerous species of and . Gastropods that naturally lack a shell, or have only an internal shell, are mostly called , and land snails that have only a very small shell (that they cannot retract into) are often called .
Snails have considerable human relevance, including as food items, as pests, and as vectors of disease, and their shells are used as decorative objects and are incorporated into jewellery. The snail has also had some cultural significance, tending to be associated with lethargy. The snail has also been used as a figure of speech in reference to slow-moving things.
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 , , and the abyssal zone depths of the sea. Although land snails may be more familiar to laymen, marine biology 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 freshwater snail.
Most snails have thousands of microscopic tooth-like structures located on a banded ribbon-like tongue called a radula. The radula works like a file, ripping food into small pieces. Many snails are herbivore, eating plants or rasping algae from surfaces with their radulae, though a few land species and many marine species are or predation . Snails cannot absorb colored pigments when eating paper or cardboard so their feces are also colored.
Several species of the genus Achatina and related genera are known as giant African land snails; some grow to from snout to tail, and weigh . The largest living species of sea snail is Syrinx aruanus; its shell can measure up to in length, and the whole animal with the shell can weigh up to . The smallest land snail, Angustopila psammion, was discovered in 2022 and measures 0.6 mm in diameter.
The largest known land gastropod is the African giant snail Achatina achatina, the largest recorded specimen of which measured from snout to tail when fully extended, with a shell length of in December 1978. It weighed exactly 900 g (about 2 lb). Named Gee Geronimo, this snail was owned by Christopher Hudson (1955–79) of Hove, East Sussex, UK, and was collected in Sierra Leone in June 1976.
Snails are Protostome. That means during development, in the gastrulation phase, the blastopore forms the mouth first. Cleavage in snails is spiral holoblastic patterning. In spiral holoblastic cleavage, the cleavage plane rotates each division and the cell divisions are complete. Snails do not undergo metamorphosis after hatching. Snails hatch in the form of small adults. The only additional development they will undergo is to consume calcium to strengthen their shell. Snails can be male, female, Hermaphrodite, or Parthenogenesis so there are many different systems of sexual determination.
Generally, land snails are most active at night due to the damp weather. The humid nighttime air minimizes water evaporation and is beneficial to land snails because their movement requires mucus, which is mostly composed of water. In addition to aiding movement, mucus plays a vital role in transporting food from the gill to the mouth, cleansing the mantle cavity, and trapping food before ingestion.
A shell-less animal is much more maneuverable and compressible, so even quite large land slugs can take advantage of habitats or retreats with very little space, retreats that would be inaccessible to a similar-sized snail. Slugs squeeze themselves into confined spaces such as under loose bark on trees or under stone slabs, logs or wooden boards lying on the ground. In such retreats they are in less danger from either predators or desiccation. Those are often suitable places for laying their eggs.
Slugs as a group are far from Monophyly; scientifically speaking "slug" is a term of convenience with little taxonomic significance. The reduction or loss of the shell has evolved many times independently within several very different lineages of gastropods. The various taxa of land and sea gastropods with slug morphology occur within numerous higher taxonomic groups of shelled species; such independent slug taxa are not in general closely related to one another.
The decollate snail ( Rumina decollata) will capture and eat garden snails, and because of this it has sometimes been introduced as a biological pest control agent. However, this is not without problems, as the decollate snail is just as likely to attack and devour other gastropods that may represent a valuable part of the native fauna of the region.
As well as being eaten as gourmet food, several species of land snails provide an easily harvested source of protein to many people in poor communities around the world. Many land snails are valuable because they can feed on a wide range of agricultural wastes, such as shed leaves in banana plantations. In some countries, giant African land snails are produced commercially for food.
Land snails, freshwater snails and sea snails are all eaten in many countries. In certain parts of the world snails are fried. For example, in Indonesia, they are fried as satay, a dish known as sate kakul. The eggs of certain snail species are eaten in a fashion similar to the way caviar is eaten.
In Bulgaria, snails are traditionally cooked in an oven with rice or fried in a pan with vegetable oil and red paprika powder. Before they are used for those dishes, however, they are thoroughly boiled in hot water (for up to 90 minutes) and manually extracted from their shells. The two species most commonly used for food in the country are Helix lucorum and Helix pomatia.
Snails and slug species that are not normally eaten in certain areas have occasionally been used as famine food in historical times. A history of Scotland written in the 1800s recounts a description of various snails and their use as food items in times of plague. (Also quoted here.
Snails were widely noted and used in divination. The Greeks poet Hesiod wrote that snails signified the time to harvest by climbing the stalks, while the Aztec mythology moon god Tecciztecatl bore a snail shell on his back. This symbolised rebirth; the snail's penchant for appearing and disappearing was analogised with the moon. Keong Emas (Javanese and Indonesian for Golden Snail) is a popular Javanese culture folklore about a princess magically transformed and contained in a golden snail shell. The folklore is a part of popular Javanese Panji cycle telling the stories about the prince Panji Asmoro Bangun (also known as Raden Inu Kertapati) and his consort, princess Dewi Sekartaji (also known as Dewi Chandra Kirana).
In contemporary speech, the expression "a snail's pace" is often used to describe a slow, inefficient process. The phrase "snail mail" is used to mean regular postal service delivery of paper messages as opposed to the delivery of email, which can be virtually instantaneous.
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