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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).
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.
The species is known as an agricultural and garden pest, an edible delicacy, and occasionally a household pet. In French cuisine, it is known as petit gris, and is served for instance in Escargot à la Bourguignonne. The practice of rearing snails for food is known as heliciculture. For purposes of cultivation, the snails are kept in a dark place in a wired cage with dry straw or dry wood. Coppiced wine-grape vines are often used for this purpose. During the rainy period the snails come out of hibernation and release most of their mucus onto the dry wood/straw. The snails are then prepared for cooking. Their texture when cooked is slightly chewy.
There are a variety of snail-control measures that gardeners and farmers use in an attempt to reduce damage to valuable plants. Traditional pesticides are still used, as are many less toxic control options such as concentrated garlic or wormwood solutions. Copper metal is also a snail repellent, and thus a copper band around the trunk of a tree will prevent snails from climbing up and reaching the foliage and fruit.
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 species of gastropods that may represent a valuable part of the native fauna of the region.
Skin creams derived from Helix aspersa snails are sold for use on wrinkles, scars, dry skin, and acne. A research study suggested that secretions produced under stress by Helix aspersa might facilitate regeneration of wounded tissue.
Because of 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.
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.
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.
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, Italy, 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.
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:
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