Most Amphibia are cold-blooded, four-legged vertebrates. Their life cycle usually begins as an egg. This is followed by an underwater gill-breathing larval stage. The final stage is as an adult with that breathes with lungs.
The first amphibians appeared about 370 million years ago. They most likely evolved from lobe finned fish.
Their respiratory system consists of primitive lungs. Respiration also occurs through the skin which permits gas exchange with water. In this way, amphibians can remain underwater and still breathe. Their circulatory (heart and arteries) and nervous systems (brain, spinal cord and nerves) are similar to most other vertebrates.
The class Amphibia belongs to the phylum Chordata. There are three major orders of amphibians within this class. These are:
Anura: include the frogs and the toads. Frogs tend to be those with smooth skin. Toads have a rougher skin with a wart like appearance. This order of animals have long rear legs with shorter fore legs. Their toes are webbed, they lack tails as adults, have large eyes and moist skin.
Caudata: include the salamanders and newts. This order resembles lizards. Members of this order have skins without scales, lack claws and have flattened tails.
Gymnophiona: include the caecilians. Caecilians lack limbs and resemble snakes or worms. Their eyes are rudimentary and these amphibians often live underground.
Amphibia are found world-wide. They live in a wide range of habitats. This includes land based, soil based, forest based and aquatic habitats. They are often nocturnal creatures of the night.
Amphibia and Humans
Amphibia are affected by human civilizations. Localized extinctions have occurred all over the world. Extinctions have been blamed on habitat destruction, changes in the climate, pollutants and introduction of other species.
Some species are poisonous to humans.
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