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1630s, "having two modes of existence, of doubtful nature," from Greek amphibia, neuter plural of amphibios "living a double life," from amphi- "of both kinds" (see amphi-) + bios "life" (see bio-).
Formerly used by zoologists to describe all sorts of combined natures (including otters and seals), the biological sense "class of animals between fishes and reptiles that live both on land and in water" and the noun derivative both are first recorded 1835. Amphibia was used in this sense from c.1600 and has been a zoological classification since c.1819.
A cold-blooded, smooth-skinned vertebrate of the class Amphibia. Amphibians hatch as aquatic larvae with gills and, in most species, then undergo metamorphosis into four-legged terrestrial adults with lungs for breathing air. The eggs of amphibians are fertilized externally and lack an amnion. Amphibians evolved from lobe-finned fish during the late Devonian Period and include frogs, toads, newts, salamanders, and caecilians.
Our Living Language : Amphibians, not quite fish and not quite reptiles, were the first vertebrates to live on land. These cold-blooded animals spend their larval stage in water, breathing through their gills. In adulthood they usually live on land, using their lungs to breath air. This double life is also at the root of their name, amphibian, which, like many scientific words, derives from Greek. The Greek prefix amphi- means "both," or "double," and the Greek word bios means "life." Both these elements are widely used in English scientific terminology: bios, for example, is seen in such words as biology, antibiotic, and symbiotic.