In these the young of some present almost exactly the adult conditions of the others, and the larvæ of the lower forms have been known to undergo transformations that have changed them from the likeness of their parents to the fashions of the allied species. Thus the adult sirens are scarcely distinguishable from the aquatic larvæ of the salamanders. Siren larvæ have developed into the condition of adult salamanders, while the young of the salamanders greatly resemble tadpoles, or larval toads and frogs. The adult menobranch has a skull like that of a tadpole, or a salamander in the larval state. In fact, wherever the young of one of these families diverge from the form of their parents, it is simply to trench upon the form of some of the allied species. We have noted that frogs and toads have the tongue fastened at the front of the mouth, and hanging free down the throat. Some of the salamanders have the tongue fastened in the middle of the under side, so that it resembles a broad, flat mushroom on a short, thick stem, while the teeth are set in a fine brush above it on the roof of the mouth. In the sirens the tongue is free in front; there are no teeth on the jaws, and the jaws are cased in a horny sheath like a beak; but down the throat there are two patches of sharp little teeth pointed backwards. Let us take a look at a siren. At first one might call it a fish or an eel, wearing queer plumes at the back of the head, and also endowed with little legs and feet. This is the “lizard-formed siren,” and the largest specimen ever found was about three feet long: most of them do not exceed twenty inches in length. The body is long and slim, like that of an eel; the skin is smooth and nearly black, besprinkled with little white dots. From the shoulders, near the head, come a pair of small, feeble legs, the feet being divided into four fingers. These feet aid the movements of the creature on the sand or mud when it emerges from the water, and serve to elevate the neck a little in catching and swallowing prey. The motion of the siren in the water is sinuous, like that of an eel; the legs may help a little as paddles, but the action from side to side, such as we have all noticed in a water-snake, is that by which it progresses. Well, let us look, for the hind-legs, now that we have seen the front ones. Where are they? Gone altogether! Nature seems to have concluded that a little beast which would let its front legs dwindle away from want of use, might as well be deprived of the hind ones entirely.