Amphibians are a class of about 3000 species of tetrapod vertebrates. Amphibians have limbs and generally spend part of their time on land, but they do not have the adaptations to an entirely terrestrial existence found in most other modern tetrapods (amniotes).
Traditionally the amphibians are taken to include all tetrapods that are not amniotes. Recent amphibians all belong to a single subgroup of these, called the Lissamphibia. Recently there has been a tendency to restrict the class Amphibia to the Lissamphibia, i.e. to exclude tetrapods that are not closer related to modern forms than they are to modern reptiles, birds, and mammals.
There are three modern orders:
Authorities disagree on whether Salientia is a Superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. In effect Salientia includes all the Anura plus a single Triassic proto-frog species, Triadobatrachus massinoti[?]. Practical considerations seem to favour using the former arrangement here and now.
For the purpose of reproduction all amphibians are bound to the water. Several species have adapted to arid and semi-arid environments, but most of them need water to lay their eggs. The larvae breathe with exterior gills. After hatching they start to transform gradually to the adult's appearance. This process is called metamorphosis. Finally the larva can leave the water and has become the adult amphibian, which is able to live on the land.
There are several exceptions from this general way of reproduction, but most amphibians are developing this way. The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on the land. But there are several other changes:
- The gills are replaced by other respiratory organs, e.g. lungs.
- The skin changes and develops glands avoiding the dehydration of the organism
- The eyes get eyelids and adapt to vision outside the water
- An eardrum is developed to lock the middle ear
