Gastropoda

Gastropods (/ˈɡæstrəpɒdz/; previously known as Univalves; class Gastropoda /ɡæsˈtrɒpədə/) are a vast and diverse group of invertebrates within the phylum Mollusca, comprising the animals commonly known as snails and slugs. With an estimated 65,000 to 80,000 living species, they form the second-largest animal class after the insects. The fossil record of gastropods extends back to the Late Cambrian. As of 2017^[update], 721 families are recognized—476 extant (some with fossil representatives) and 245 extinct known only from fossils.

Gastropods inhabit an extraordinary range of environments, including marine, freshwater, and terrestrial ecosystems. They occur in gardens, woodlands, deserts, mountains, rivers, lakes, estuaries, mudflats, intertidal zones, the deep sea, hydrothermal vents, and even in parasitic niches.

The term snail generally refers to gastropods with a large external shell into which the body can fully retract, while slugs have no shell or a small internal one, and semislugs can only partially withdraw into their reduced shell. Marine gastropods include familiar forms such as abalones, conches, periwinkles, whelks, and cowries, whose shells are usually coiled in adulthood. In contrast, limpets and related groups coil their shells only in the larval stage, retaining a simple conical form as adults.

Because of their vast diversity, gastropods exhibit remarkable variation in anatomy, behavior, feeding, and reproduction, making broad generalizations difficult. Gastropods remain one of evolution’s most successful and adaptable animal lineages.

In the scientific literature, gastropods were described as "gasteropodes" by Georges Cuvier in 1795. The word gastropod comes from Greek γαστήρ (gastḗr 'stomach') and πούς (poús 'foot'), a reference to the fact that the animal's "foot" is positioned below its guts.

The earlier name "univalve" means one valve (or shell), in contrast to bivalves, such as clams, which have two valves or shells.

At all taxonomic levels, gastropods are second only to insects in terms of their diversity.

Gastropods have the greatest numbers of named mollusk species. However, estimates of the total number of gastropod species vary widely, depending on cited sources. The number of gastropod species can be ascertained from estimates of the number of described species of Mollusca with accepted names: about 85,000 (minimum 50,000, maximum 120,000). But an estimate of the total number of Mollusca, including undescribed species, is about 240,000 species. The estimate of 85,000 mollusks includes 24,000 described species of terrestrial gastropods.