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Any member of a class (Amphibia) of cold-blooded vertebrate animals that includes more than 6,200 species in three groups: frogs and toads (order Anura), salamanders (order Caudata), and caecilians (order Gymnophiona). Probably evolved from certain fish species of the Early Devonian Period (416–397 million years ago), amphibians were the first vertebrates to move from an aquatic environment to land. Most species have an aquatic larval, or tadpole, stage that metamorphoses into a terrestrial adult, but a few species spend their entire life in water. Amphibians are found worldwide, the majority in the tropics.
This entry comes from Encyclopædia Britannica Concise. For the full entry on amphibian, visit Britannica.com.