Gastropods (35000 species) are asymmetrical molluscs with a well-developed head bearing a pair of tentacles with an eye at the base of each, and a broad flattened foot. Evolution of the shell has involved conversion from a shield to an asymmetrical and spiral/helical portable retreat. This twisting of the shell is not associated with the phenomenon known as ‘torsion’. Torsion occurs as the two sides of the body develop unevenly with a twisting of the visceral hump. It still occurs in most larval forms as they metamorphose into adults. As a result the gills and anus lie in front of the visceral hump. Sea slugs undergo de-torsion and the gills and anus of the adult are behind the visceral hump.
Gastropod shells are very varied. The first type was like a hose coiled on the ground e.g. Planorbis. Later changes led to an increase in height and aperture size, accompanied by spiralling to form a series of whorls wrapped around a central columella. Many groups have reduced shells or no shells at all.
Gastropods feed in a multitude of ways: as herbivores, carnivores, scavengers, suspension feeders, deposit feeders and parasites. The radula has become highly adapted to grate, rasp, brush, cut grasp or convey food. Although gastropods retain the flat, creeping, ancestral foot, most are relatively active and mobile animals.
The eggs of some gastropods develop into trochophores. These usually develop into a veliger larva with a head, foot mantle and vibrating cilia for feeding and locomotion. Both stages are planktonic and serve for dispersal.
There are three subclasses:
Prosobranchs are generally marine animals and breathe by means of gills. The shell is made up of a series of whorls at some stage of the life cycle. There are three orders: Archaeogastropoda, Mesogastropoda, and Neogastropoda.
In sea slugs and their relatives (Opisthobranchia), the shell is reduced or absent. These gastropods are often brightly coloured with external gills and brightly coloured. They undergo torsion followed by de-torsion.
Snails and slugs (Pulmonata) are gastropods in which the mantle cavity is modified to form a lung for breathing air. It may be further modified for breathing under water. Pulmonates undergo torsion and usually bear shells, which lack an operculum. Most are terrestrial or fresh water animals; a few are marine. Pulmonate molluscs are thought to have descended from prosobranch snails living in brackish water e.g. Bithynia tentaculata.
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