Common Frog, showing tongue in action.
There is a common belief that at a certain time the tail of the tadpole falls off. Nature is not so wasteful. This tail, when it has served its purpose as a swimming organ—that is to say, as soon as the hind legs have developed enough to take up their duties—is gradually absorbed.
And this fact recalls another. It will be remembered that it was pointed out that for some time after leaving the egg no food was taken at the mouth, because there was no mouth, but life was sustained by the reserve of yolk within the body, the remains of the egg, in short. Similarly, we have a second period when no food is taken, and this takes place while the tail is being used up, and the mouth is being transformed. Exactly how this using-up process is effected cannot be easily explained here; but it forms what is known as a reserve store of food. In a similar way, dormice, squirrels, and bears grow very fat before they retire to some snug hole to sleep out the long winter. The gradual waste of the body which goes on during the long sleep is made good by slowly using up the fat which was accumulated during the summer and autumn.
At last, then, the tail of the tadpole disappears, and with this several new features become apparent. These are the new breathing arrangements, a new mouth and system of catching food, and shorter intestines.
About this new breathing. In ourselves this is done by means of our ribs, which alternately rise, increasing the cavity of the chest and the capacity of the lungs, and fall, or rather are pulled down, decreasing the chest cavity, and pressing out the air from the lungs. The frog pumps in air by that curious movement of the throat which the ignorant suppose to be a preparation for poison-spitting. When the throat is depressed the mouth cavity is increased, and air rushes in through the nostrils and fills the chamber. When the floor of his mouth is raised again the cavity is reduced, and the air is forced down the windpipe into the lungs, being unable to escape through the nostrils, because they are closed by special valves.
The mouth is now toothless, and of great size. The young frog feeds on living prey, which is generally caught by the tongue. For this purpose, the tongue in the frog and toad is fixed to the front of the floor of the mouth, so that the tip of the tongue points backwards towards the throat! In capturing, say a fly, the frog creeps as near his prey as he can manage, and then, with a lightning movement, darts the tongue forward on the unsuspecting victim. The tongue being covered with a sticky substance, the fly adheres to the trap and is drawn in a twinkling into the cavern, from which return is impossible. The working of the tongue may be seen in the illustration.
The shortening of the intestine follows in consequence of the change to more nutritious diet. In the young tadpole it is long and may be seen coiled up like a watch-spring through the skin of the abdomen; in the adult these coils disappear.
Such, then, is the brief outline of the life-history of one of Nature's water babies. We have traced it from the egg to the grown-up form: and here we must stop, though all that is of interest does not end here. I could tell you of the curious way in which the frog changes colour to suit his surroundings; of how he changes his skin; of his wonderful vocal powers, and a hundred other things. But meanwhile, try and discover it for yourselves by keeping a few frogs as pets, starting, as I did, with the spawn taken from a ditch in spring.
W. P. Pycraft, F.Z.S., A.L.S.