Gastropods twist 180 degrees—and outnumber every animal class but insects
Snails and slugs form class Gastropoda, a mollusk lineage whose living roster is pegged somewhere between sixty-five and eighty thousand species—second only to insects. Fossils reach the Late Cambrian; as of 2017 some 721 families are recognized, 476 still living. Georges Cuvier coined gasteropodes in 1795 for the stomach-foot layout.
They occupy gardens, deserts, trenches, hydrothermal vents, and parasitic niches. Named mollusks include about 24,000 terrestrial gastropods; marine estimates near 30,000 and freshwater about 4,000 to 5,000, while only 0.0001 percent of deep-sea floor has been biologically surveyed. Since 1500, 444 gastropod species have gone extinct, eighteen survive only in captivity, and sixty-nine are possibly extinct. Fossil species number at least 15,000. Continental slopes hold peak marine diversity; shelves and abyssal plains are poorer.
Torsion rotates the visceral mass 180 degrees in development so the anus sits above the head—a shift separate from shell coiling and partly reversed in many opisthobranchs. Asymmetry erases or reduces right-side organs. Most shelled forms coil dextrally with an operculum trapdoor; slugs reduce or lose the shell, especially on calcium-poor acidic soils where conchiolin films replace thick carbonate. Desert snails such as Sphincterochila boissieri cope with aridity; others live 10,000 meters down.
Heads usually bear two or four tentacles with eyes ranging from light-dark ocelli to lensed pits; gastropods have no hearing. Land species lean on smell; marine opisthobranchs use rhinophores. A radula scrapes food; siphons let predators taste water. Most marine kinds breathe with gills and hemocyanin, while many freshwater and most land forms use a pallial lung—Planorbidae uniquely deploy hemoglobin. Many land snails are hermaphrodites that fire love darts in courtship. Diversity defeats neat generalities, which is precisely the clade's evolutionary boast.
Source: Gastropod
