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The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to large. There are many thousands of species of sea snails and sea slugs, as well as freshwater snails, freshwater limpets, land snails and land slugs.

The class Gastropoda contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian. There are 611 families of gastropods known, of which 202 are extinct and appear only in the fossil record.

Gastropoda (previously known as univalves and sometimes spelled "Gasteropoda") are a major part of the phylum Mollusca, and are the most highly diversified class in the phylum, with 60,000 to 80,000 living snail and slug species. The anatomy, behavior, feeding, and reproductive adaptations of gastropods vary significantly from one clade or group to another. Therefore, it is difficult to state many generalities for all gastropods.

The class Gastropoda has an extraordinary diversification of habitats. Representatives live in gardens, woodland, deserts, and on mountains; in small ditches, great rivers and lakes; in estuaries, mudflats, the rocky intertidal, the sandy subtidal, in the abyssal depths of the oceans including the hydrothermal vents, and numerous other ecological niches, including parasitic ones.

Although the name "snail" can be, and often is, applied to all the members of this class, commonly this word means only those species with an external shell big enough that the soft parts can withdraw completely into it. Those gastropods without a shell, and those with only a very reduced or internal shell, are usually known as slugs; those with a shell into which they cannot withdraw are termed limpets.

The marine shelled species of gastropod include edible species such as abalone, conches, periwinkles, whelks, and numerous other sea snails that produce seashells that are coiled in the adult stage - though in some, the coiling may not be very visible, for example in cowries. In a number of families of species, such as all the various limpets, the shell is coiled only in the larval stage, and is a simple conical structure after that.