Geological formations furnish no ancestral connection between the Stegocephalia and modern salamanders; but the limbless, wormlike, burrowing and blind cæcilians of the tropics exhibit certain stegocephalian characteristics, especially a scaly skin, which put them into a division (Apoda) by themselves. The remainder of the class, that is, Amphibia (also called Batrachia) in general, have a soft, moist, naked skin, and are naturally divisible into two orders:
1. Urodela—Tailed amphibians: newts and salamanders.
2. Anura—Tailless amphibians: frogs and toads.
Modern amphibians in general are animals fitted for life both on land and in water. All are born from eggs hatched in water, and the young, at first in a larval form unlike the adult condition, have external gills adapted to breathing in that element; but in most cases they lose their gills, and as adults acquire lungs for breathing air. This metamorphosis of the young, comparable to that of the nymph-producing insects, is the especial characteristic of the class. The skeleton is of the vertebrate model, but in the Urodela is largely cartilaginous. The skin is smooth, soft, moist, and covered only with a filmy coat of horny texture that is molted from time to time as the animal needs room to grow. The skin abounds in sense organs about the head and along the sides of the body—an inheritance from the lateral line of fishes—which are most active in the larvæ, and disappear altogether with age in most frogs and toads, although they revive in salamanders in the breeding season.
The skin also contains many mucus glands and other larger glands, especially on the back. These emit under provocation a poisonous liquid that is fatal to small animals, and very irritating to the eyes, nose, and throat of larger ones. Most, if not all, Amphibia, says Dr. Gadow, are more or less poisonous, and it is significant that many of the most poisonous exhibit a very conspicuous yellow or orange upon a dark ground, which is so widespread a sign of poison. There is no venom in their bite—in fact, their teeth are too small, although numerous, to let anyone fear their biting. The skin is heavily laden with pigment, and this is displayed in many amphibians in striking patterns of bright coloring. Certain groups possess in a high degree the power of altering their colors to conform to their surroundings.
An interesting feature of the amphibians is that power of repairing mutilations of the body and replacing lost parts which is so well known in worms, hydroids, and other lowly creatures, and is termed "regeneration." This ability is most active in young specimens. Tadpoles frequently have their tails bitten off, whereupon new ones grow quickly. Salamanders fight bitterly, tearing off each other's gills and limbs, and turtles and fishes frequently bite off their tails. New tails are generated speedily, and usually in good and effective form, although they contain no regenerated caudal vertebræ, but only a rod of cartilage. The ability to rebuild lost parts is much less among the frogs.
Another notable fact is that here for the first time we meet with a voice organ, and a real voice expressing emotions, although in an extremely limited way. This is most noticeable in the tree frogs, which are the most advanced of the Amphibia in organization.
NEWTS AND SALAMANDERS
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.