The Urodela are represented throughout the whole northern hemisphere except in desert regions, as far in North America as southern Canada, and also southward to Panama; and in the Old World, northward to the line of very cold winters and southward to the Mediterranean and Indo-China. In the main, however, our genera are different from those of Europe and Asia.

The largest and best known of American urodeles is a member of the family Proteidæ and genus Necturus, and is widely known as "water dog" or "mud puppy," because of the doglike shape of its head. It is a brown, robust creature, sometimes two feet long, with bushy gills, retained throughout its life, springing from open gill clefts in three bright red tufts on each side of the head. It inhabits cold, rapid streams, hiding under stones by day, and moving about at night in search of crawfish, worms, insect larvæ, frogs, etc., and dodging hungry snapping turtles. But little smaller, and even more ugly in appearance, is the "hellbender," representing the family Amphiumidæ. These blackish creatures are to be found in mountainous regions, and hide during the day under loose rocks. By the time they are about three years old their gills have been absorbed, and their lungs are in service, so that they are compelled to rise to the surface occasionally for drafts of air. They hunt at night for food, preferring crawfish and, fishermen say, fish eggs. The breeding habits of this animal have only lately become known, and Mr. B. G. Smith, who has made a special investigation of them, says that the breeding season begins (in Pennsylvania) in August, when hellbenders of both sexes come out more freely from their rock shelters and roam about, frequently in small companies. The small number of eggs produced are hidden in a pocket under a loose stone; and the young, which are more like tadpoles than the form of their parents, breathe by gills which do not completely disappear until the animals have reached nearly their maturity.

Otherwise our salamanders are small species found in brooks, ponds, and wet woods, and often getting into cellars and wells. Uninformed persons think them to be lizards, and foolishly fear them, but except for the irritation of the hands that may follow rough handling they are utterly harmless to man or his property, and serve him by devouring great quantities of insects and worms.

A common species in damp, neglected woodlands is the little red-backed fellow that is so light and leaping in its movements when disturbed, even throwing off its tail in its panic of fear. It is more terrestrial than most, laying its few eggs in rotting wood instead of going into the water for that purpose; and the young carry gills but a few days. This red-backed Plethodon must not be confused with the small newts, bright vermilion with a row of glowing spots along the sides, that are found in woods in summer. They are young specimens of Diemyctylus viridescens, which is common all over the eastern part of the United States and southern Ontario. The parents are green, and wholly aquatic in habits. The larvæ have gills and swim about until early autumn, by which time their gills have been gradually absorbed, and they go ashore, where their coats change in color from a mottled green to scarlet. This red condition and their residence on land continue until the autumn of the third, or the spring of the fourth year of their lives, when they become sexually mature, resume a greenish dress, go back to the water, and pass the rest of their lives there.

Mention can be made of only one more species—the black, yellow-spotted "tiger triton," which is themost widely spread and often seen of our terrestrial salamanders. It is especially noteworthy because of the extraordinary condition of suspended development exhibited by its larva, the famous edible axolotl of Mexican lakes, which, while still retaining larval gills and aquatic habits, grows nearly or quite to the size of its parents—three to four inches—and becomes capable of breeding. Similar cases are known in certain lakes in southern Europe; and it appears that this arrested development, together with natural growth of body, occurs occasionally in many other amphibians. The condition is termed "neotony," but the biological explanation of it is not clear.