The word snail is a common name that can be used for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. (Snails lacking a shell or having only a very small one are usually called slugs. Snails that have a broadly conical shell that is not coiled, or appears not to be coiled, are often known as limpets.) When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails.
The class of Gastropoda (the snails and slugs) is second only to the insects in terms of total number of species. Snails are extraordinarily diverse in habitat, form, behavior, and anatomy, and therefore what is true of one snail species may not at all be true of another.
Snails can be found in a wide range of different and alike environments from ditches to deserts to the abyssal depths of the sea. The great majority of snail species are marine. Many others are terrestrial, and numerous kinds can be found in fresh water, and even brackish water. Many snails are herbivorous, though a few land species and many marine species are omnivores or predatory carnivores.
Although the average person might perhaps be more familiar with terrestrial snails, land snails are in the minority. Marine snails have much greater diversity, and a greater biomass. Snails which respire using a lung belong to the group Pulmonata, while those with gills form a paraphyletic group, in other words, snails with gills are divided into a number of taxonomic groups that are not very closely related.
Snails with lungs and with gills have diversified widely enough over geological time that a few species with gills can be found on land, numerous species with a lung can be found in freshwater, and a few species with a lung can be found in the sea.
Although the word snail is often used for all shelled gastropods, the word "snail" can also be used in a much more limited sense, to mean various larger species of air-breathing (pulmonate) land snails. The majority of this article is about air-breathing land snails.
0 comments
Post a Comment