| Gastropods | ||||||
|---|---|---|---|---|---|---|
| Scientific classification | ||||||
| ||||||
| Orders | ||||||
|
Subclass Prosobranchia Archaeogastropoda Mesogastropoda Neogastropoda Subclass Opistobranchia Cephalaspidea Anaspidea Sacoglossa Thecosomata Gymnosomata Notaspidea Nudibranchia Pteropoda Subclass Pulmonata Systellomatophora Basommatophora Stylommatophora | ||||||
The gastropods, or univalves, are the largest and most successful class of molluscs, with 60,000-75,000 species, and second largest class of animals, with over 100,000 species, comprising the snails and slugs, as well as a vast number of marine and freshwater species. They typically have a well-defined head with two or four sensory tentacles, and a ventral foot, which gives them their name (Greek gaster, stomach, and poda, feet). They are distinguished by torsion, a process where the body coils to one side during development. Most members have a shell, which is in one piece and typically coiled or spiralled that usually opens on the right hand side. Several species have an operculum that operates as a trapdoor to close the shell. This is usually made of a horny material, but in some molluscs it is calcareous. In some members, the slugs, the shell is reduced or absent, and the body is streamlined so its torsion is relatively inconspicuous.
While the best-known gastropods are terrestrial, more than two thirds of all species live in a marine environment. Marine gastropods include herbivores, detritus feeders, carnivores and a few ciliary feeders, in which the radula is reduced or absent. The radula is usually adapted to the food that a species eats. The simplest gastropods are the limpets and abalones, both herbivores that use their hard radulas to rasp at seaweeds on rocks. Many marine gastropods are burrowers and have siphons or tubes that extend from the mantle and sometimes the shell. These act as snorkels, enabling the animal to continue to draw in a water current containing oxygen and food into their bodies. The siphons are also used to detect prey from a distance. These gastropods breathe with gills, but some freshwater species and almost all terrestric species have developed lungs. While the gastropods with lungs all belong to one group (Pulmonata), the gastropods with gills are paraphyletic.
Sea slugs are often flamboyantly coloured, either as a warning if they are poisonous, or to camouflage them on the corals and seaweeds on which many of the species are found. Their gills are often in a form of feathery plumes on their backs which gives rise to their other name, nudibranchs. Nudibranchs with smooth or warty backs have no visible gill mechanisms and respiration may take place directly through the skin. A few of the sea slugs are herbivores and some are carnivores. Many have distinct dietary preferences and regularly occur in association with certain species.