SNAIL. In England the word " snail " in popular language is associated with Gasteropods which inhabit land or fresh water and which possess large conspicuous spiral shells ; terrestrial Gasteropods in which the shell is rudimentary and concealed are distinguished as "slugs." In Scotland the word " slug " is absent from the vernacular vocabulary, both shell-bearing and shell-less inland molluscs being known as snails. Marine Gasteropods are occasionally termed "sea-snails," and the compounds "pond-snails," "river-snails," " water-snails " are in common use. The commonest land-snails are those species which constitute the family Ifelicidx, order Pulmonata, sub-order Stylommatophora. The other two families of the same sub-order, Limacidre and Onchidiidx, include all the slugs. In the first of these are comprised all the slugs known in Great Britain, and indeed in Europe. The Onchidiithu are entitled to the name "sea-slugs," as they are shell-less Pulmonates living on the seashore, though not actually in the sea. The term " water-snails " includes the whole of the remaining sub-order of the Pulmonata, namely, the Basommatophorct, in which the eyes are sessile. This division comprises two families, Linznxida3 and Avriculidx ; some of the members of the first are amphibious, some entirely aquatic ; the snails of the second family are found near but not in the water. Thus the whole of the Pulmonata which breathe air, are destitute of gill-plumes and operculum, and have a complicated hermaphrodite reproductive system are either snails or slugs. But there are a considerable number of snails, both terrestrial and aquatic, which are not Pulmonates. The land-snails which have no gill-plume in the mantle-chamber and breathe air, but have the sexes separated, and possess an operculum belong to the order Azygobranchia, of which they form a distinct sub-order, the Pneunionochlanzycla, containing three families, C yclostomidte, 11Clicinidx, and Acicvlidx. The fresh-water snails -which are not Pulmonates are the Paludinidte, Valvatidw, and Ampullaridx, together with 1Veritina, a genus of the Xeriticln. These all possess a fully developed gill-plume and are typical Azygobranchiates of the sub-order Holochlamyda, most of the members of which are marine.

The family Helieidx has a world-wide distribution. In Helix the spire forms a more or less obtuse-angled cone ; there are above 1200 species, of which 24 are British. Helix nem,. ails , L., of which 11. hortensis is a variety, is one of the commonest forms. Helix pomatia, L., is the largest species, and is known as the "edible snail" ; it is commonly eaten in France and Italy, together with other species. It was formerly believed to have been introduced into Britain by the Romans, but there is no doubt that it is a native. In Succinea the cone of the spire is acute-angled ; three species are British. In Vitrina the spire is very flat and the surface glassy. In Butinius the spire is elongated with a pointed apex. Pupa is named from its resemblance to a chrysalis, the apex being rounded. The shell of Clausilia is sinistral and its aperture is provided with a hinged plate. The commoner European slugs of small size all belong to the genus Limax, in which the opening of the mantle-chamber is posterior. L. fiamts is the cellar slug. L. agrestis, L. arborum, L. maximus' occur in gardens and fields. The larger black slugs are species of Arlon, of which two are British, A. ater and A. hortensis. Testacella haliotidea is common in Great Britain and throughout Europe.

The Livinwidte occur in all parts of the world. Linin.zus contains the largest species. L. pereger, Muller, is ubiquitous in Great Britain and common all over Europe. All the species are usually infested with Cercarix and Redix, the larval forms of Trematodo parasites of vertebrates. L. truntatulus harbours the Ccrcaria of Fasciola hepatica, the liver-fluke, which causes rot in sheep. -4ncylus, which occurs in rivers, has a minute limpet-like shell. Planorbis has the spire of the shell in one plane. Physa is smaller than Li77171£7211,9 and has the upper part of the spire much shorter. In the Auriculidx the aperture is denticulated. Auricula is confined to the East Indies and Peru. Carychium minimum is British.

Of the Cyclostomiche only one species, Cyclostoma clegans, Muller, is British ; it hides under stones and roots. The HelicinidEe are exotic, ranging from the West Indies to the Philippines. Of the Aciculidn, which are all minute, Acicula lineata is British.

The Ampullarithe are confined to the tropics. Ampullaria has very long tentacles and a long siphon formed by the mantle.

Valvata is common in fresh waters throughout Britain ; the gill when the animal is expanded is protruded beyond the mantle-chamber. The Paludinidx are common in the northern hemisphere. Paint/but and Bitleynia are both British genera. In Paladin the whorls of the spiral are very prominent ; the genus is viviparous. Bithynia is smaller and the shell smoother.

Neritina has a very small spire, the terminal portion of the shell containing nearly the whole animal.

For the morphology and classification of snails, see Mom.uscA, vol. xvi. p. 64S sq. A history of the British forms is given in Gwyn Jeffreys's British Coachology, 1562, and by Forbes and Hanley in British No/it/sea. For specie-graphical details, see Woodward's Manual of the Mollusca, 1875, and Bronn's Th.terreich (Weichthiere). For Fasciola hepatica, see Thomas, Quart. Jeers. Mic. Set., 1582.