The gastropods, or univalves, are the largest and most successful class of mollusks, 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 (as viewed with the shell apex pointing upward). 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.
The first gastropods were exclusively marine, such as the Aldanella, Helcionella and Scenella, which are found in rocks of the early Cambrian period. By the Ordovician period the gastropods were a varied group present in a range of aquatic habitats. Commonly fossil gastropods from the rocks of the early Palaeozoic era are too poorly preserved for accurate identification. Fossil gastropods are less common during the Palaeozoic era than bivalves.
Most of the gastropods of the Palaeozoic era belong to primitive groups, a few of which still survive today. By the Carboniferous period many of the shapes we see in living gastropods can be matched in the fossil record, but despite these similarities in appearance the majority of these older forms are not directly related to living forms. It was during the Mesozoic era that the ancestors of many of the living gastropods evolved.
One of the earliest know terrestrial (land-dwelling) gastropods is Maturipupa which is found in the Coal Measures of the Carboniferous period in Europe, but relatives of the modern land snails are rare before the Cretaceous period when the familiar Helix first appeared.
In rocks of the Mesozoic era gastropods are slightly more common as fossils, their shell often well preserved. Their fossils occur in beds which were deposited in both freshwater and marine environments. The "Purbeck Marble" of the Jurassic period and the "Sussex Marble" of the early Cretaceous period which both occur in southern England are limestones containing the tightly packed remains of the pond snail Viviparus.
Rocks of the Cenozoic era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms. The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves.
Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these trails are of debatable origin, some of them do resemble the trails made by living gastropods today.
Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods. An example of this is Bellerophon from the limestones of the Carboniferous period in Europe which may be mistaken for a cephalopod.
Gastropods are one of the groups that record the changes in fauna caused by the advance and retreat of the Ice Sheets during the Pleistocene period.
The taxonomy of the Gastropoda is under constant revision, but more and more the old taxonomy is being abandoned. Nevertheless terms as 'opisthobranch' and 'prosobranch' are still being used in a descriptive way (and no longer as taxons). In a sense, we can speak of a taxonomic jungle when we go down to the lower taxonomic levels. The taxonomy of the Gastropoda can be different from author to author. But with the arrival of DNA-sequencing, a more definite taxonomy of the higher taxonomic levels is to be expected in the near future.
Till recently there were four subclasses. :
Prosobranchia (gills in front of the heart).
Opisthobranchia (gills to the right and behind the heart).
Gymnomorpha (no shell)
Pulmonata (with lungs instead of gills)
According to the newest insights (Ponder & Lindberg, 1997), the taxonomy of the Gastropoda should be rewritten. According to these authors, taxa can only be valid when defined in cladistic terms. In their opinion, a classification with a rigid set of hierarchical levels is not necessary or even desirable. Their thorough morphological analysis led to several cladistic trees, producing a single cladistic tree. The authors then provided names for the clades in this tree.
Integrating their findings into a working taxonomy will be a true challenge in the coming years.
At present, it is impossible to give a classification of the Gastropoda that has consistent ranks and also reflects current usage.
Next is a proposed classification, down to the level of superfamily.