This class, containing over 35,000 living and 15,000 fossil gastropod species, comprises the largest class of Mollusca, and includes the limpets, top shells, periwinkles, slipper shells, snails, slugs, sea hares, abalones, nudibranches, or sea slugs, and sea butterflies. Gastropods are primarily marine, but freshwater and terrestrial forms occur. When present, the typical gastropod shell is a three-layered, spiral whorl of calcium carbonate, which varies in color, shape, ornamentation, and size according to the species. Within this shell is the tall, coiled body mass. Some forms, such as slugs, are shell-less and do not have a tall body mass. Gastropod larvae undergo a twisting, or torsion, that brings the rear of the body (mantle cavity, gills, and anus) to a position near the head and results in the twisting of internal organ systems. In many this twisted form is retained by the adult; in others it is partially lost.
There are three subclasses: the Prosobranchia, which contains the majority of gastropods; the Pulmonata, which contains the land snails; and the Opisthobranchia, which includes the sea hares and sea slugs. The latter subclass consists of animals with reduced shells or none at all. Most gastropods are motile, but some, e.g., the slipper shell ( Crepidula ), are sedentary. Some, such as the sea butterflies, swim, and others, including the terrestrial snails, move by means of a well-developed foot.
Many gastropods are herbivores, or plant eaters, with multitoothed radulas for scraping algae from various substrata. Among the carnivorous, or animal-eating, species is the conch, which feeds on smaller mollusks, and the cone shells ( Conus ), which feed on fish and annelid worms that they first paralyze with poison contained in their hollow radula teeth. The poison is also toxic to humans, causing paralysis and sometimes death. Gastropods have a complex nervous system with ganglia.
Reproduction is variable, but most gastropods have separate sexes. Fertilization of the egg occurs in seawater. Some gastropods are hermaphrodites (having both sexes in the same individual) and some are protandric hermaphrodites, i.e., they are male first and become female as they age.
Gastropods are economically valuable as food for many animals, including humans. Some gastropods are serious pests; the common slug, for example, causes much garden damage.
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