Amphibia

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).

Classification

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:

   * Order Urodela or Caudata (salamanders): 350 species
   * Order Anura (frogs and toads) (in Superoder Salientia): 3,700 species
   * Order Gymnophiona or Apoda (caecilians): 170 species

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.

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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