SNAIL, the name usually given to land molluscs of the class Gastropoda which have spiral shells, such as the common snail (Helix asperse) and the wood snail (Cepea nemoralis). In Scot land both shell-bearing and shell-less land molluscs are known as "snails." Marine Gastropod molluscs are sometimes called "sea snails" and the freshwater gastropods (Vivipara, Limnaea, etc.) are known as "pond snails" and "river snails." The shell-bearing Gastropoda ("snails" in the wide sense) are a very large class of animals, embracing some 40,000 living and extinct species, with a world-wide distribution.
Perhaps the most striking feature in the structure of snails is their shell. In the majority this is spiral and consists of several whorls which have a right-handed (dextral) coil. A left-handed (sinistral) coil is found in certain forms, either as an occasional variation of normally dextral forms (Limnaea pereger and Nep tunea antique) or as a fixed generic characteristic (Physa, Clau silia). Sometimes the shell is cup-shaped (Patella, the common limpet), tubular (Coecum) or plate-like (Scutum). In many land and marine Gastropods it becomes internal and degenerate or is eventually lost entirely, and a slug-like form is attained. The land snails belong, with some exceptions, to the order Pulmonata and a very large proportion of them are placed in the family Helicidae, which is one of the largest groups of land invertebrates. They are mainly animals of retiring habits living on green plants or on decaying vegetable debris, though a few (e.g., Glandina, Strep taxis) are carnivorous. As a rule, they are more frequently found upon calcareous soils and are certainly rare on "acid" formations.
They usually live during the daytime buried out of sight under leaves and, as a certain amount of moisture is necessary for their well-being, they aestivate in hot and sunny weather in crevices or under ground, from which protection they emerge at night or during rain.
Including the "snail-slugs" (Testacella), there may be said to be 125 species of land and fresh, or brackish, water snails in the British Isles. The largest family (indeed, the largest in the phylum Mollusca) is the Helicidae, of which there are 22 species in Great Britain. Among the commonest forms are Helix asperse, the common or speckled snail, Cepea nemoralis and hortensis, the wood and garden snails and Limnaea peregra, the common pond snail. Certain species (e.g., Belgrandia marginate), which are still living in continental Europe, are found in a fossil state in the British Isles. Paludestrina jenkinsi, the only mollusc at present known to reproduce itself parthenogenetically (without fertilization), is found very plentifully in Great Britain.
In North America there are very numerous species of fresh water and land snails. Among representatives of the former group are some 65 species of Limnaea and 25 species of Planorbis. Of the land snails important families are the Pupillidae, with 5o species ; the Zonitidae, with 65 species ; and the exceedingly numerous Helicidae, of which the exclusively American genus Polygyra alone contains 125 species. (Concerning the use of snails by man, see GASTROPODA.)