amphibian: one of the 8 classes of the subphylum Vertebrata. Amphibians represent the evolutionary bridge between (vertebrate) jawed fish and (vertebrate) reptiles, whereby it is speculated that intermediary lobe-finned fishes, through gradual, transitional stages, including the tetrapod-like fish, Tiktaalik (375 mya), became increasingly terrestrial, and thereby less dependent on an aquatic environment for oxygen intake (respiration, breathing) and egg-laying. Amphibia include frogs, toads, newts, salamanders, and caecilians (limbless amphibians, that resemble large worms). Their features are cold-bloodedness (body heat derived from external sources); aquatic larvae (young must breed in water, as opposed to amniotes, where extra-embryonic membranes provide a land-based aquatic environment for the embryo); gill breathing in young; but lung and moist glandular skin breathing in semi-terrestrial adults (from multiple bibliographic sources).