Summary
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.
Glossary
Acanthostega
one of the earliest known tetrapods
Amphibia
frogs, salamanders, and caecilians
Anura
frogs
Apoda
caecilians
caecilian
legless amphibian that belongs to the clade Apoda
cutaneous respiration
gas exchange through the skin
frog
tail-less amphibian that belongs to the clade Anura
salamander
tailed amphibian that belongs to the clade Urodela
tadpole
larval stage of a frog
Urodela
salamanders