Gastropod literally means "stomach in the foot". A group (formerly a class) of molluscs (eumollusca, conchifera, strombidae) characterised notably by a fleshy mass called a foot used to creep along or swim. The head has one or two pairs of tentacles and two eyes located on stalks.
In this very diverse group (103,000 species), the size and shape of species are extremely variable. Animals measure from 0.1 mm to 60 cm. They live on land (snail, slug), in the marine environment, or in fresh water. Some species are parasites.
The shell, when present, is always in one piece (as opposed to bivalve molluscs such as the mussel or oyster) and dorsal, most often spiral-shaped, but may also be straight (limpet or "Chinese hat"). The shell may be very thick, such as cowrie shells (Cypraea), or on the other hand, reduced to a delicate strip covered by the mantle (a tunic unique to molluscs, which covers the inner organs and secretes the shell, when it exists). This is the case of the genus Limax (land slug) and aplysia (sea hare). Some gastropods do not have a shell, such as the Arion (red slug).
A few known species: snails, slugs, limpets, aplysia, winkles, cowries, cones, whelks, abalone (sea ears)...