Amphibia
Amphibia is one of the five major classes of vertebrates. There are three orders in the amphibia group, two of which are widely familiar, frogs (Anura) and salamanders (Caudata), and one of which is less well-known, the tropical caecilians (Gymnophiona). The name "amphibian" refers to the use of both aquatic and terrestrial habitats, and the life history patterns of species in the group.
Amphibian Characteristics and Life History
The earliest tetrapods (four-legged, terrestrial vertebrates) were amphibians, and living amphibian species retain some of the primitive characteristics of the first terrestrial vertebrates, which invaded land habitats during the geologic period known as the Devonian, which was approximately 408 million years ago. For example, unlike other terrestrial vertebrates, amphibians lack scales and claws, and are instead characterized by a moist, glandular skin composed of living cells. The skin is involved in respiration to some degree in most amphibians. Certain lungless salamanders rely largely on the skin for gas exchange. In many species, skin glands secrete noxious, or sometimes highly poisonous, substances that serve as deterrents to predators. This is sometimes associated with a warning coloration that advertises toxicity, and some species have even evolved specific defensive postures to show off effectively their warning coloration.
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