Gastropods are a large and diverse group of marine, terrestrial, freshwater, or parasitic mollusks, containing some 70.000 species, devided over the subclasses Prosobranchia (mainly marine), Opistobranchia (marine), and Pulmonata (largely terrestrial or limnic). They share the common feature that during development the visceral mass is rotated through 180 degrees in an anticlockwise direction.
The univalve shell, which is generally present, is calcareous, and apically closed. It is usually spirally coiled with a variable number of whorls, rarely solute, sometimes cap-shaped. The columellar axis is hollow or solid, the columellar muscle primitively paired; sometimes horseshoe-shaped, this muscle is lost in shell-less forms.
The usually asymmetrical animal is basically divided into head, foot. and visceral mass; the latter contains most organ systems. The head in unspecialized forms has a pair of cephalic tentacles, and eyes are usually associated with them; both tentacles and eyes can be lost; an additional pair of tentaclelike cephalic structures is sometimes present. The mouth opening is simple or modified as a proboscis, commonly variably retractile. The distal buccal cavity has cartilages (jaws) and a radula with a variable number of backward-directed teeth arranged in rows and series; there is one or more pairs of salivary glands which empty into the buccal cavity and which may sometimes be accessory poison glands; salivary glands are not always present. The esophagus is variable: wide and glandular in prosobranchs (sometimes with esophageal glands) and simpler in opisthobranchs and pulmonates. The stomach has an associated digestive gland: the intestine is short to long and convoluted.
The foot has a creeping sole, and sometimes is modified for swimming or lost; primitively there are lateral grooves between the foot and mantle, forming an epipodium with tentacles and integumentary sensory organs, usually flat, sometimes round and more or less elongate; the foot is sometimes broadened laterally or extended medially for swimming or sometimes reduced or separated into a propodium, metapodium, and parapodia. In prosobranchs, the foot usually bears an operculum in the adult, which is mostIy present in the ontogeny of all groups; the operculum is calcareous. corneous, or both.
The integumentary mantle forms the shell and covers the dorsum and body, and usually forms a separate mantle or pallial cavits in which, primitively, are located respiratory organs (ctenidia), chemosensory structures (osphradia), and mucoid hypobranchial glands; the ctenidia may be lost or replaced with secondary gills or with a vascularized pulmonary airbreathing sac; the mantle edge in higher taxa forms a siphonal process to draw in and expel water; the mantle, like the shell, can be lost.
Ctenidia primitively are paired and bipectinate, and directed anteriorly; secondarily, the right ctenidium is lost and the left variably altered to the monopectinate condition; in terrestrial forms, the ctenidia are lost and a vascularized pulmonary sac is developed; in other taxa, secondary respiratory structures can be elaborated.
Gastropoda are primitively gonochoristic with the gonoduct in association with the right kidney which, when lost, facilitates elaboration of structures for forming or storing eggs or semen; in advanced taxa, the male has an intromittent reproductive organ (primitively not retractile), mostly near the right tentacle and of cephalic or pedal orgin; the spermduct is an open groove or closed tube; most opisthobranchs and pulmonates have a retractile penis generally on the right, sometimes with an armature of spines or stylets; some prosobranchs and nearly all opisthobranchs and pulmonates are hermaphroditic; fertilization is primitively external, and advanced forms have internal fertilization; eggs sometimes are laid singly, or multiply in rows and/or encapsulated; these mollusks are oviparous or ovoviviparous, often larviparous.
Largely adopted from Parker, S.P. (ed.), 1982. Synopsis and classification of the living organisms, McGraw-Hill, Vol. 1: 960-961.