Mud-puppies, salamanders, etc. (Urodela).—Technical Note.—If possible obtain specimens of mud-eels (Siren), common in the South, or mud-puppies (Necturus), common in the central North, as examples of batrachians with gills persisting in the adult stage. One or more species of Amblystoma may be found in almost any part of the country, and larvæ of large size may be found with the external gills. For an example of the general long-tailed or Urodelous type of batrachian any salamander or newt occurring in the vicinity of the school may be used. The little green triton or eft (Diemictylus viridiscens) of the eastern States, or its larger brown-backed congener of the Pacific coast (D. torosus) is common in water, while another eft, the little red-backed salamander, (Plethodon) is common in the woods under logs and stones. The external characters of the body should be compared with those of the toad. The skeleton should be prepared by macerating away the flesh (for directions, see p. [452]), and the presence of the many caudal vertebræ and the ribs, the equality in size of the legs, and other points should be noted. Compare with skeleton of toad. Make drawings. It will be well, also, to dissect out and examine the various internal organs of the salamander, comparing them with the same organs in the toad. The salamander, indeed, is in many ways better than the toad as an example of the class. Its body is less adaptively modified and shows the essentially fish-like character of the batrachian structure.
The batrachians which retain external gills in the adult stage are the members of two families of which the American representatives are known as mud-eels (Siren) and mud-puppies or water-dogs (Necturus). The mud-eels, which are found "in the ditches in the swamps of the southern States from South Carolina to the Rio Grande of Texas and up the Mississippi as high as Alton, Illinois," are blackish in color, have no hind legs and are long and slender, with the tail shorter than the rest of the body. They reach a length of nearly three feet. The mud-puppies, found in the Great Lakes and in the rivers of the upper Mississippi valley, are brown with colored spots, and are about two feet long when full grown. They have both fore and hind legs.
A few salamanders, while not possessing external gills when adult, have a spiracle or small circular opening in the side of the neck which leads into the throat. The best-known American salamander of this kind is the large heavy-bodied blackish water-dog or "hellbender" (Cryptobranchus) of the Ohio River. It is about two feet long, and is "a very unprepossessing but harmless creature." It has a conspicuous longitudinal fold of skin along each side of the body. The largest known batrachian, the giant salamander of Japan (Megalobatrachus), reaching a length of three feet, is related to the water-dog.
Of all the salamanders the most interesting are the blunt-nosed salamanders (Amblystoma). A dozen or more species of Amblystoma occur in North America, of which tigrinum, a dark-brown species with many irregular yellow blotches sometimes arranged in cross-bands, is the most widespread. The larvæ of some Amblystoma retain their gills until they have reached a large size, and in one or two species the usual metamorphosis is very long delayed and the salamanders produce young while in the larval condition, that is, while retaining the gills and a compressed fin-like tail. In the case of a certain Mexican species (A. maculatum) it is believed that the final metamorphosis never occurs. The Mexicans call these gilled larval Amblystoma axolotls, and use them for food. For a long time naturalists supposed the Amblystoma larvæ which produce young to be the adults of a species of salamanders which retained their gills through life, like the sirens and mud-puppies, and classified them in a distinct genus.
Fig. 121.—The Western brown eft, or salamander, Diemyctylus torosus. (From living specimen.)
Of the various common salamanders or newts some are found in streams, ponds, and ditches, and some under logs and stones in the woods. The aquatic forms have the tail compressed (flattened from side to side), while the land forms have the tail cylindrical, tapering to a point. Most of the land-salamanders produce their young alive, while the water forms lay eggs which are usually attached to a submerged plant-stem. The salamanders are, almost without exception, found only in the northern hemisphere.
Frogs and toads (Anura).—There are about a dozen species of frogs in the United States. The largest of these, and indeed the largest of all the frogs, is the well-known bullfrog (Rana catesbiana), which reaches a length (head to posterior end of body) of eight inches. It is found in ponds and sluggish streams all over eastern United States and in the Mississippi valley. It is greenish in color with the head usually bright pale green. Its croaking is very deep and sonorous. The pickerel-frog (R. palustris), which is bright brown on the back with two rows of large oblong square blotches of dark brown on the back, is found in the mountains of eastern United States. The little pale reddish-brown wood-frog (R. sylvatica) with arms and legs barred above is common in damp woods and is "an almost silent frog." The peculiar and infrequently seen frogs known as the "spade-foots" (Scaphiopus) are subterranean in habit and usually live in dry fields or even on arid plains and deserts. They pass through their development and metamorphosis very rapidly, appearing immediately after a rain and laying their eggs in temporary pools. At this time of egg-laying they utter extraordinarily loud and strange cries. Some frogs in other parts of the world live in trees, and the eggs of one species are deposited on the leaves of trees, leaves which overhang the water being selected so that the issuing young may drop into it.