As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of salamanders and all caecilians are limbless. The most important characteristic of extant amphibians is a moist, permeable skin used for cutaneous respiration. The fossil record provides evidence of amphibian species, now extinct, that arose over 400 million years ago as the first tetrapods. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of frogs, like the majority of amphibians, consists of two distinct stages: the larval stage and metamorphosis to an adult stage. Some species in all orders bypass a free-living larval stage.
one of the earliest known tetrapods
frogs, salamanders, and caecilians
legless amphibian that belongs to the clade Apoda
gas exchange through the skin
tail-less amphibian that belongs to the clade Anura
tailed amphibian that belongs to the clade Urodela
larval stage of a frog