Amphibians

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Amphibians are a class of tetrapod vertebrates, which are a link between fish and the true land-living vertebrates like reptiles, birds and mammals.

Classification

All recent amphibians belong to the subclass Lissamphibia, while the other subclasses are extinct. There are three orders within the Lissamphibia:

• Order Urodela or Caudata (salamanders)
• Order Anura (frogs and toads) (in Superoder Salientia)
[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.]
• Order Gymnophiona or Apoda (caecilians)

Reproduction

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

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