Snail

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 Helicidae, order Pulmonata, sub-order Stylommatophora. The families Limacidae, Arionidae and Oncidiidae of the same sub-order, include nearly all the slugs. The Oncidiidae 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 Basommatophora, in which the eyes are sessile, with the exception of the Auriculidae. The latter are terrestrial and occur mostly near the seashore. 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 orders Aspidobranchia and Pectinibranchia, and constitute the families Helicinidae, Proserpinidae, Hydrocenidae, Cyclophoridae, Cyclostomatidae and Aciculidae. The fresh-water snails which are not Pulmonates are the Paludinidae, Valvatidae and Ampullaridae, together with Neritina, a genus of the Neritidae. These all possess a fully developed gill-plume and are typical Pectinibranchs of the sub-order Taenioglossa, most of the members of which are marine.

The family Helicidae 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 nemoralis, L., of which H. 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 Bulimus 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. flavus is the cellar slug. L. agrestis, L. arborum, L. maximus occur in gardens and fields. The larger black slugs are species of Anion, of which two are British, A. ater and A. hortensis. Testacella haliotidea is common in Great Britain and throughout Europe.

The species of Helix are all herbivorous, like the Pulmonata generally; snails and slugs are well-known enemies to the gardener. The animals being hermaphrodite copulate reciprocally. The eggs of Helix are laid separately in the earth, each contained in a calcified shell; those of Limax are also separate, but the shell is gelatinous. Helix hibernates in a torpid condition for about four months, and during this period the aperture of the shell is closed by a calcareous membrane secreted by the foot.

The Limnaeidae occur in all parts of the world. Limnaeus contains the largest species. L. pereger, Muller, is ubiquitous in Great Britain and common all over Europe. All the species are usually infested with Cercariae and Rediae, the larval forms of Trematode parasites of vertebrates. L. truncatulus harbours the Cercaria of Fasciola hepatica, the liver-fluke, which causes rot in sheep. Ancylus, which occurs in rivers, has a minute limpet-like shell. Planorbis has the spire of the shell in one plane. Physa is smaller than Limnaeus and has the upper part of the spire much shorter. In the Auriculidae the aperture is denticulated. Auricula is confined to the East Indies and Peru. Carychium minimum is British.

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

The Ampullaridae 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 Paludinidae are common in the N. hemisphere. Paludina and Bithynia are both British genera. In Paludina 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 Gastropoda. A history of the British forms is given in Gwyn Jeffreys's British Conchology (1862), and by Forbes and Hanley in British Mollusca. For speciegraphical details, see Woodward's Manual of the Mollusca (1875), and Bronn's Tierreich (Weichtiere). For Fasciola hepatica, see Thomas, Quart. Journ. Mic. Sci. (1882).