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Snail is a common name that is applied most often to land snails, terrestrial pulmonate gastropod molluscs. However, the common name "snail" is also applied to most of the members of the molluscan class Gastropoda that have a coiled 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 thousands of species of sea snails and freshwater snails. Occasionally a few other molluscs that are not actually gastropods, such as the Monoplacophora, which superficially resemble small limpets, may also informally be referred to as "snails".
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 surfaces with their radulae, though a few land species and many marine species are omnivores or predatory carnivores.
Several species of the genus Achatina and related genera are known as giant African land snails; some grow to 15 in (38 cm) from snout to tail, and weigh 1 kg (2 lb). The largest living species of sea snail is Syrinx aruanus; its shell can measure up to 90 cm (35 in) in length, and the whole animal with the shell can weigh up to 18 kg (40 lb).
Types of snails by habitat 
Gastropod species that lack a conspicuous shell are commonly called slugs rather than snails. Some species of slugs have a reduced shell, some have only an internal vestige that serves mainly as a calcium repository, and others have no shell at all. Other than that there is little morphological difference between slugs and snails. There are however important differences in habitats and behaviour. 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, and often those also are suitable places for laying their eggs.
Slugs as a group are far from monophyletic; biologically 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.
Human relevance 
In agriculture 
Certain species of land snails are prized as food, and are farmed for human consumption in some parts of the world, see heliciculture. However it is also true that some land snails impact agriculture as pests. Although most species are harmless, certain species of snails and slugs damage crops by feeding on roots, leaves, stems and fruits. These species are able to penetrate plant material using their abrasive radula. Metaldehyde-containing baits are frequently used for snail control, though they should be used with caution, as they are toxic to dogs and cats.
Cultural depictions 
Due to its slowness, the snail has traditionally been seen as a symbol of laziness. In Christian culture, it has been used as a symbol of the deadly sin of sloth. Psalms 58:8 uses snail slime as a metaphorical punishment.
Divination and other religious uses 
Snails were widely noted and used in divination. The Greek poet Hesiod wrote that snails signified the time to harvest by climbing the stalks, while the Aztec 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.
Love darts and Cupid 
Professor Ronald Chase of McGill University in Montreal has suggested the ancient myth of Cupid's arrows might be based on early observations of the love dart behavior of the land snail species Helix aspersa.
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.
Snails as food 
As well as being relished 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 a number of countries (principally Spain, Philippines, Morocco, Nigeria, Algeria, Cameroon, France, Sicily, Portugal, Greece, Belgium, Vietnam, Laos, Cambodia, Cyprus, Ghana, Malta, Terai of Nepal, southwestern China, Northeast India states such as Manipur, Tripura and parts of the USA). 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.
Famine food 
Snails and slug species that are not normally eaten in certain areas have occasionally been used as famine food in historical times. Variants of the following event have occurred in Europe from time to time:
- In a popular publication quoted below occurs the following notice of a well-known land mollusk, in connection with a traditionary story of the plague, which has long had general currency in Scotland: ‘In the woodlands, the more formidable black nude slug, the Arion or Limax, will also be often encountered. It is a huge voracious creature, herbivorous, feeding, to Barbara’s astonishment, on tender plants; fruits, as strawberries, apples; and even turnips and mushrooms; appearing morning and evening, or after rain; suffering severely in its concealment in long droughts, and remaining torpid in winter. The gray field slug (Limax agrestis) is actually recommended to be swallowed by consumptive patients! In the town of Dundee there exists a strange traditionary story of the plague, connected with the conversion, from dire necessity of the Arion ater, or black slug, to a use similar to that which the luxurious Romans are said to have made of the great apple-snail. Two young and blooming maidens lived together at that dread time, like Bessie Bell and Mary Gray, in a remote cottage on the steep (indeed almost perpendicular) ascent of the Bonnetmaker’s Hill. Deprived of friends or support by the pestilence that walked at noonday, they still retained their good looks and healthful aspect, even when the famine had succeeded to the plague. The jaundiced eyes of the famine-wasted wretches around them were instantly turned towards the poor girls, who appeared to thrive so well whilst others were famishing. They were unhesitatingly accused of witchcraft, and had nearly fallen a prey to that terrible charge; for betwixt themselves they had sworn never to tell in words by what means they were supported, ashamed as they felt of the resource to which they had been driven; and resolved, if possible, to escape the anticipated derision of their neighbours on its disclosure. It was only when about to be dragged before their stern inquisitors, that one of the girls, drawing aside the covering of a great barrel which stood in a corner of their domicile, discovered, without violating her oath, that the youthful pair had been driven to the desperate necessity of collecting and preserving for food large quantities of these Limacinae, which they ultimately acknowledged to have proved to them generous and even agreeable sustenance. To the credit of the times of George Wishart—a glimpse of pre-reforming enlightenment—the explanation sufficed; the young women escaped with their lives, and were even applauded for their prudence.
See also 
- Fredericks, Anthony D. (2010-03-31). How Long Things Live. Stackpole Books. p. 73. ISBN 9780811736220. Retrieved 19 June 2012.
- "Pests in Gardens and Landscapes". University of California, Davis. Retrieved 2010-08-03.
- de Vries, Ad (1976). Dictionary of Symbols and Imagery. Amsterdam: North-Holland Publishing Company. p. 430. ISBN 0-7204-8021-3.
- Jack Tresidder, Symbols and Their Meanings, New York: Barnes & Noble, 2006, ISBN 978-0-7607-8164-7, p. 41.
- Cooper, JC (1992). Symbolic and Mythological Animals. London: Aquarian Press. p. 213. ISBN 1-85538-118-4.
- "Lovebirds and Love Darts: The Wild World of Mating". news.national-geographic.com. National Geographic Society. Retrieved 2010-02-21.
- Chambers, Robert. Domestic annals of Scotland, from the reformation to the revolution. Pub: W. & R. Chambers 1858. May be downloaded from: http://archive.org/details/domesticannalsof02chamiala quoted at http://www.electricscotland.com/history/domestic/vol2ch2c.htm
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- Introduction to Snails, Infoqis Publishing, Co.