(Last Updated on : 21/08/2014)
Snails form the largest group of molluscs with only one shell. There are many kinds, varying widely in habits and structure. There are the true lung-breathing land snails, fresh-water snails and sea snails. All are widely distributed throughout the world. Snails are present in a diverse range of environments which includes deserts, ditches and the abyssal depths of the sea. Even if people are more familiar with the terrestrial snails, the land snails form the minority. The marine snails form a huge majority of snail species and possess a large diversity and a bigger biomass. A large variety of snails can also be seen in fresh water. Several snails are herbivorous; though some land species and a large number of marine species are predatory carnivores or omnivores.
One of the different varieties of snails is the Land Snail. They are very much familiar and are marked by the slime-coated, trailing body which comes out of a roundish spiral shell. The animal moves so slowly that in English 'a snail's pace' means very slowly indeed. A snail has two pairs of tentacles on its head; a short pair in front and a long pair a little behind. The hinder pair has an eye at the tip of each tentacle. Snails can contract and completely withdraw these tentacles into a hollow in the head. Like the Land Snails, the Pond and River Snails are also lung-breathing. But in these molluscs the eyes are situated at the base of the tentacles, of which there is a single pair. The tentacles are capable of contraction but cannot be completely withdrawn into the hollow of the head. Two genera of Pond snails are common. They are Lymnaea and Planorbis. The former has an elegant, conically spired shell. It is known for its worldwide distribution and unlimited capacity for adapting itself to physical extremes. One species is recorded from a tarm situated at over 5000 metres elevation in the Himalaya and another from a depth of nearly 250 metres in the lake of Geneva. It also survives in polluted waters, sulphur springs, brackish water and even in the hot-water geysers of Iceland. Although it has reacquired the habit of living in water, Lymnaea's lung is still functional, so that visits to the surface are necessary for the snail to obtain air.
Sea Snails are also rich in several kinds of snails which vary very widely in shape, habits and other details. There are the Top and Turban Shells, Perwinkles and violet snails. Top shells and Turban Shells are familiar univalve molluscs of all seashores, living on mud-covered rocks of the intertidal region. In their internal structural details they agree with the primitive gastropods like Limpets. They possess a radula, carrying numerous tiny teeth for grinding sea plants. Their shells are fairly heavy, usually brightly coloured, ornamented outside and pearly within. Species like Trochus radiatus and Clanculus depictus have conically spiral shells carrying rows of small smooth tubercles over the whods and coloured designs of red and greyish blue. They have a flat base and their shell-lids are thin and made of lime. Turbo is a large turban-shaped shell with rounded whorls, brownish or greenish in colour, rounded at the base and having a circular, thick, hemispherical upper column. The shells of Turbo have commercial value for the mother-of-pearl contained in them.
Snails breathing with the help of lungs belong to the Pulmonata group and which breathe with the help of gills are in the paraphyletic group. It can also be said that snails having gills are segregated into a several taxonomic groups which are not closely related to each other while the snails possessing lungs and gills are scattered hugely over geological time that lesser species having gills are seen on land, several species having lungs are seen in freshwater while a very small number of species possessing lungs are seen in the sea. Snails possess thousands tooth-like structures which help them to rip off the food into pieces.