Anatomy of a frog and some other examples of amphibians.
An amphibian is a vertebrate organism of the class Amphibia, which includes frogs, toads, salamanders, and some types of newts, having an aquatic early stage (e.g., tadpole) and developing air-breathing lungs as an adult (e.g., frog). There are three living orders: Urodela (newts and salamanders), Anura (frogs and toads), and the little-known Apoda (the caecilians). Some 3,000 species have been identified, distributed worldwide. Additionally, there are two fossil groups: Lepospondyla and Labyrinthodonta.
In contrast to fish, amphibians have a smooth, scale-less skin (except for some caecilians), which plays a role in respiration. The paired limbs are jointed structures, and though the basic number of fingers is five, many amphibians have less than this. Living on land, their body has to be supported (the weight of water-dwelling creatures is supported by the water) and the backbone acts as a girder transmitting the weight to the four legs.
When moving quickly, newts wriggle in a way that resembles that of fish, but slow movement is brought about by the movement of the legs, the body being raised up off the ground. In frogs the skeleton and musculature are specialized for jumping and swimming – the number of vertebrae is much reduced. Adult frogs have paired lungs. Only the tadpoles have gills, though some adult salamanders retain their gills.
Amphibians of temperate regions commonly hibernate because they are cold-blooded and become sluggish at low temperatures.