AMPHIBIANS.
CHAPTER VI.
FROGS AND TOADS.
Photo by W. P. Dando, F.Z.S.] [Regent's Park.
AMERICAN BULL-FROG.
Young ducks are a favourite food of this voracious Batrachian.
The Amphibian Class, through the Newts and Salamanders more especially, would appear at first sight to have much in common with and to be most closely allied to the Lizards, previously described. As a matter of fact, however, the group is much more nearly related to the Fishes. Quite the most characteristic feature in the Amphibians that is indicative of the above-mentioned affinity is the circumstance that for a more or less longer period of their existence their respiratory organs take the form of external gills, structures not found in any of the preceding vertebrate classes. Another diagnostic character of the Amphibia is afforded by the circumstance that they all pass through a transitional or larval condition before arriving at the adult state. The familiar tadpole phase of the common frog or toad typically illustrates this point. During its earliest larval state the fish-like resemblance is especially conspicuous. In addition to possessing gills, the body is limbless, and produced into a long fish-like tail, having superior and inferior fin-like membranes, with which the little animal propels itself through the water. These locomotive fins, however, are never furnished with supporting fin-rays, as obtains among the Fishes. In contradistinction to the Lizards and Snakes, the skin of Amphibians is never covered with spines or scales, but is soft and naked. In many of the Toads and Salamanders the surface of the skin is, however, warted and highly glandular, and capable of emitting an acrid and sometimes poisonous fluid. More or less pronounced conditions of moisture are essential for the well-being of all Amphibians. The eggs are deposited, and the earlier or larval conditions, with but few exceptions, passed, in the water, while the adults remain in its near proximity, and frequently take up their abode in it. Amphibia do not, however, drink water after the manner of lizards and other reptiles, but absorb all the moisture they require through the surface of their skins. The deeper and more essential skeletal elements of the Amphibia differ conspicuously from those of the preceding groups. The vertebræ in the permanently gill-bearing species more particularly are scarcely to be distinguished from those of fishes. In the Frog and Toad Tribe, on the other hand, they are reduced to a less number, seven or eight only, than is found among any other vertebrates, while ribs do not exist or are rudimentary and functionless throughout the class. Many bones of the skull in the Amphibia, as well as its general construction, are more in accord with those of fishes than of ordinary reptiles. The tongue, not always present, is attached immediately inside the front of the lower jaw, its tip pointing down the animal's throat. It is remarkable that, notwithstanding their aquatic proclivities, no Amphibian has been discovered which frequents salt water.
