Gastropods (/ˈɡæstrəpɒdz/), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (/ɡæsˈtrɒpədə/).

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, 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.

[2] 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.

[16] Gastropods are found in a wide range of aquatic and terrestrial habitats, from deep ocean trenches to deserts.

Other snails have adapted to an existence in ditches, near deepwater hydrothermal vents, in oceanic trenches 10,000 meters (6 miles) below the surface,[17] the pounding surf of rocky shores, caves, and many other diverse areas.

[18] Snails are distinguished by an anatomical process known as torsion, where the visceral mass of the animal rotates 180° to one side during development, such that the anus is situated more or less above the head.

The gill-combs, the olfactory organs, the foot slime-gland, nephridia, and the auricle of the heart are single or at least are more developed on one side of the body than the other.

This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior.

[citation needed] Gastropods typically have a well-defined head with two or four sensory tentacles with eyes, and a ventral foot.

Some gastropods have adult shells which are bottom heavy due to the presence of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture which may be important for gravitational stability.

This serves either as a warning, when they are poisonous or contain stinging cells, or to camouflage them on the brightly colored hydroids, sponges, and seaweeds on which many of the species are found.

[citation needed] In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, nudibranchs.

[citation needed] The primary organs of excretion in gastropods are nephridia, which produce either ammonia or uric acid as a waste product.

Again, in some land snails, an unusual feature of the reproductive system of gastropods is the presence and utilization of love darts.

[citation needed] Courtship is a part of the behavior of mating gastropods with some pulmonate families of land snails creating and utilizing love darts, the throwing of which have been identified as a form of sexual selection.

In some species that have evolved into endoparasites, such as the eulimid Thyonicola doglieli, many of the standard gastropod features are strongly reduced or absent.

Some predatory carnivorous gastropods include: cone shells, Testacella, Daudebardia, turrids, ghost slugs and others.

Studies based on direct observations, fecal and gut analyses, as well as food-choice experiments, have revealed that snails and slugs consume a wide variety of food resources.

Additionally, they feed on fungi, lichens, algae, soil, and even other animals, both living and dead, including their feces.

[27] Despite their ecological importance, there is a notable lack of research exploring the specific roles that terrestrial gastropods play within soil food webs.

[26] Many terrestrial gastropod mollusks are known to consume fungi, a behavior observed in various species of snails and slugs across distinct families.

[28][29] Notable examples of fungivore slugs include members of the family Philomycidae, which feed on slime molds (myxomycetes), and the Ariolimacidae, which primarily consume mushrooms (basidiomycetes).

[29] Snails have also been reported to feed on penny buns as well as Coprinellus,[33] Aleurodiscus, Armillaria, Grifola , Marasmiellus, Mycena, Pholiota, and Ramaria.

This variability stresses the diverse dietary adaptations among slug species and their ecological roles in fungal consumption.

[38] Early Cambrian species such as Helcionella, Barskovia, and Scenella are no longer considered gastropods, and the small coiled Aldanella from the same period is probably not even a mollusk.

[39] Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand.

[citation needed] Gastropods also provide important evidence of faunal changes during the Pleistocene epoch, reflecting the impacts of advancing and retreating ice sheets.

Consensus has not been reached yet considering the relationships at the very base of the gastropod tree of life, but otherwise the major groups are known with confidence.

Convergent evolution, which appears to exist at especially high frequency in gastropods, may account for the observed differences between the older phylogenies, which were based on morphological data, and more recent gene-sequencing studies.