Urodela

One of the orders of the class Amphibia, also known as the Caudata. The members of this order are the tailed amphibians, or salamanders, and are distinguished superficially from the frogs and toads (order Anura) by the possession of a tail, and from the caecilians (order Apoda) by the possession of limbs. Salamanders resemble lizards in that members of both groups normally have a relatively elongate body, four limbs, and a tail. The similarity is however, only superficial. The most obvious external difference, the moist glandular skin of the amphibian and dry scaly skin of the reptile, is underlain by the numerous characters that distinguish reptiles from amphibians. See Anura

The vast majority of salamanders are well under a foot (0.3 m) in length. The largest is the giant salamander of Japan and China, which may attain a length of 5 ft (1.5 m). A close relative, the hellbender, which lives in streams in the eastern United States, grows to over 2 ft (0.6 m) in length.

Four well-developed limbs are typically present in salamanders, but the legs of the mud eel (Amphiuma) of the southeastern United States are very tiny appendages and of no use in locomotion. The sirens (Siren and Pseudobranchus), also aquatic salamanders of the same region, have undergone even further degeneration of the limbs and retain only tiny forelimbs. Salamanders with normal limbs usually have four toes on the front feet and five on the rear, though a few have only four on all feet. A feature of the Urodela not shared with the Anura is the ability to regenerate limbs.

The moist and highly vascularized skin of a salamander serves as an organ of respiration, and in some forms shares with the buccal region virtually the whole burden. In one entire family of salamanders, the Plethodontidae, lungs are not present. Even in those forms with lungs and gills, dermal respiration plays a very important part. The tympanum is absent in salamanders and the middle ear is degenerate. Hence these animals are deaf to airborne sounds. Undoubtedly correlated with this is the reduction of voice; salamanders are mute or produce only slight squeaking or, rarely, barking sounds when annoyed. However, salamanders can detect sounds carried through ground or water.

Salamanders live in a variety of aquatic or moist habitats. Many forms are wholly aquatic, living in streams, rivers, lakes, and ponds. Others live on land most of the year but must return to water to breed. The most advanced species live out their lives in moist places on land or, in the case of some tropical species, in trees, and never go in the water.

Producing the young alive rather than by laying eggs is very rare among salamanders. The eggs of salamanders may be deposited in water or in moist places on land. Presumably, aquatic breeding is the more primitive. Fertilization is external in most of the Cryptobranchoidea, internal by means of spermatophores in the remaining suborders.

About 300 species of salamanders are known to be living today, and these are distributed among about 54 genera. The greatest concentration of families and genera of salamanders is found in the eastern United States, and these animals are one of the few major groups of vertebrate animals that is distinctly nontropical. No salamander occurs south of the Equator in the Old World, and only 18 species of the evolutionarily advanced family Plethodontidae are found as far south as South America in the New World. See Amphibia

Urodela