Chordata: Class Amphibia

Class Amphibia

The amphibians, the first vertebrates to have limbs, evolved during the Devonian period. They are only partially terrestrial: Their externally fertilized eggs are laid in freshwater, and they go through a gilled, aquatic larval stage (the tadpole stage) before metamorphosing into land-living adults. The skin of the adult is water-permeable, and the animal must live in a moist environment to prevent desiccation. The adult usually breathes by means of lungs, although some breathe directly through the skin. The heart is a three-chambered structure that creates a partial separation between oxygenated blood, destined for the body tissues, and depleted blood, destined for the lungs; this provides better oxygenation than does a system in which the two kinds of blood mix. There are only three groups of amphibians living today. The salamanders and newts are closest to the basic amphibian stock in form and in method of locomotion. Although supported by limbs, they move with a wriggling motion similar to that of a fish. The frogs and toads are specialized for jumping, with long, muscular hind legs, while the tropical caecilians are burrowing forms that have lost all but vestigial traces of their limbs.

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