Gastropods are one of the main divisions of the Phylum Mollusca. They include animals which bear a coiled or uncoiled shell, and others, called slugs which have no hard parts. Typical of the Class Gastropoda are the land and freshwater snails, the many conchs, whelks, limpets and periwinkles. Originally they were entirely marine, but especially in Mesozoic and Cenozoic times large numbers of them became adapted for life in fresh waters and on the land. A large majority of the group remained in the sea. The average size of the shells of the group is approximately 25 millimeters in length or diameter, but fully grown adults of different kinds range from 0.5 mm to approximately 60 cm.
The land snails, Helix, and the shallow water marine Buccinum are familiar examples of gastropods. In each case the animal is an elongate, flat-soled creature which carries a spirally coiled shell on its back. The tip of the spiral shell points backwards, and the opening into the largest, last-formed turn of the shell is in a forward position, directed downward.
The head is the mobile, anterior part of the body which bears the mouth, eyes and one or two pairs of tentacles. Beginning just inside the mouth is a long muscularly movable rasping mechanism which is composed of many minute horny teeth arranged in transverse rows on a tough, flexible ribbon. This mechanism, called a radula, functions for tearing food into particles. This feature has been used in classification of living gastropods.
Eyes are borne at the tips of the tentacles of some gastropods, and in others they are situated near the base of the tentacles.
The calcareous shell is the structure on which paleontological study of the group must be based. Most gastropod shells show features of form and structure which can be used in identifying species and genera. Shells of gastropod may be divided into two groups in general: those with little or no coiling and those which are partly or very distinctively coiled.
Range: Cambrian to Recent