amphibian, in zoology, cold-blooded vertebrate animal of the class Amphibia. There are three living orders of amphibians: the frogs and toads (order Anura, or Salientia), the salamanders and newts (order Urodela, or Caudata), and the caecilians, or limbless amphibians (order Gymnophiona, or Apoda), a little known tropical group. Amphibians, the most primitive of the terrestrial vertebrates, are intermediate in evolutionary position between the fish and the reptiles.
Typically amphibians undergo a metamorphosis from an aquatic, water-breathing, limbless larva (called a tadpole) to a terrestrial or partly terrestrial, air-breathing, four-legged adult. The eggs are usually deposited in water or in a protected place where their moisture will be conserved; they have neither shells nor the sets of membranes that surround the eggs of reptiles and other higher vertebrates. Some amphibians lay their eggs in dry places, and the young undergo the larval stage within the egg, emerging as small adults; in these the eggs have evolved various protective structures. Adult amphibians differ from reptiles in having moist skins, without scales or with small, hidden scales.
All living amphibians are specialized for their way of life, none representing the main amphibian stock from which the reptiles evolved. The salamanders and newts are superficially the most similar to ancestral amphibians, having long tails and front and hind legs of approximately equal size. Frogs and toads are highly modified for jumping, with large, muscular hind legs and no tails, while the caecilians have lost all external traces of limbs.
R. Carroll, The Rise of Amphibians (2009).
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