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Amphibians (class Amphibia) are a taxon of animals that include all tetrapods (four-legged vertebrates) that do not have amniotic eggs. Amphibians are ectotherms (meaning their body temperature adjusts according to their environment), and generally spend part of their time on land, but most do not have the adaptations to an entirely terrestrial life. There are 6,022 described, living species of amphibians. The study of amphibians and reptiles is known as herpetology.

Amphibians developed with the characteristics of pharyngeal slits/gills, a dorsal nerve cord, a notochord, and a post-anal tail at different stages of their life.

Throughout their history, amphibians have ranged in size from large forms, such as the 5 foot (150cm) long Eryops of the Permian period, and the Giant Japanese and Chinese Salamanders of today, down to the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus iberia (leptodactylid frog) from Cuba, with a total length of 9.6-9.8 millimeters (0.4 inches). Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic.

For the purpose of reproduction most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true sea water amphibians. Several hundred frog species in adaptive radiations, however, do not need any water whatsoever. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water.

Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, bypassing the tadpole stage entirely. Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that lives in the jelly-like layer of the eggs has evolved several times. The larvae (tadpoles or polliwogs) breathe with exterior gills. After hatching, they start to transform gradually into the adult's appearance. This process is called metamorphosis. Typically, the animals then leave the water and become terrestrial adults, but there are many interesting exceptions to this general way of reproduction.

The most obvious part of the amphibian metamorphosis is the formation of four legs in order to support the body on land. But there are several other changes:

  • The gills are replaced by other respiratory organs, i.e. lungs
  • The skin changes and develops glands to avoid dehydration
  • The eyes get eyelids and adapt to vision outside the water
  • An eardrum is developed to lock the middle ear
  • In frogs and toads, the tail disappears

Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, and disease. However, many of the causes of amphibian declines are still poorly understood, and amphibian declines are currently a topic of much ongoing research.

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