Amphibian, animal with moist, hairless skin through which water can pass in and out. Nearly all amphibians live the first part of their lives in water and the second part on land—a double life reflected in the name amphibian, which comes from the Greek words amphi, meaning “both,” and bios, meaning “life.” Amphibians were the first animals with backbones to adapt to life on land. They are the ancestors of reptiles, which in turn gave rise to mammals and birds.
Scientists recognize more than 4,000 species of amphibians, all of which are members of one of three main groups: frogs and toads, salamanders, or caecilians. Frogs and toads are the most abundant of all amphibians, numbering more than 3,500 species. Frogs have smooth skin and long limbs. Toads, in contrast, have warty skin and short limbs.
There are about 360 known species in the salamander group, which also includes newts and mud puppies. Members of this group have long, slender bodies ending in tails. Some salamanders live entirely on land, whereas others never leave the water, and still others spend some time in the water and some on land. Caecilians, with about 160 species, are the rarest of amphibians. They have no limbs and look much like earthworms. Most live underground and spend their time burrowing in the soil, but a few are aquatic.
Scientific classification: Amphibians are the only members of the class Amphibia, which contains three living orders. Salamanders, a group that includes newts and mud puppies, make up the order Caudata; frogs and toads make up the order Anura, and caecilians make up the order Gymnophiona.