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. [Illustration: FIG. 121.--The Western brown eft, or salamander, _Diemyctylus torosus_. (From living specimen.)]