CHAPTER XXIX
AMPHIBIANS
You will remember that the amphibians are distinguished from the true reptiles by having to pass through a tadpole stage before they obtain their perfect form. A good example is the frog, which in one kind or another exists in all parts of the earth except the very coldest. No doubt, you have often seen great masses of its jelly-like spawn floating on the surface of ponds early in the spring; and you must have wondered how such small creatures as frogs could possibly lay such enormous batches of eggs.
But the fact is that when these eggs are first laid they are very tiny. Each egg is only about as big as a small pin's head. Instead of having shells, however, they are covered with a very elastic skin, while at the same time they soak up water. So, as soon as they pass into the pond they begin to swell, and very soon each egg is as big as a good-sized pea.
Tadpole and Frog
In the middle of each egg is a round black spot, which increases in size every day. This is the future tadpole, and after a time the egg-skin splits, and out it tumbles into the water.
It is an odd-looking creature—just a big round head with a tiny pair of gills and a little wavy tail, and nothing else at all. But it manages to swim by wagging its tail, and it feeds on the tiny scraps of decaying matter which are always floating about in the water of the pond. Before long a little pair of legs begin to show themselves just at the base of the tail. A few days later another pair begin to grow in front of them. Then, by slow degrees, the tail passes back into the substance of the body, and so do the gills, while lungs are developed and nostrils are opened. And by the time that all these changes have taken place the tadpole has ceased to be a tadpole and has turned into a frog.
It leaves the water now and lives upon land, feeding upon small insects, which it catches in a most curious way. Its tongue is turned, as it were, the wrong way round; for the root is just inside the lips, while the tip is down the throat. Besides this, the tongue is very elastic and very sticky. So the animal catches its victims just as the chameleon does, flicking out its tongue at them and just touching them with the tip, to which they adhere. And as the tongue is drawn back into the mouth it pokes them down the throat; so that frogs do not even have to take the trouble of swallowing their dinner.
If you look at a frog's hind feet, you will notice that the toes are joined together by webbing. This allows them to be used in the water as well as upon dry land. It is generally said that frogs swim. But if you watch them in the water you will see at once that they do not really swim at all, but leap along, just as they leap along the ground. And each leap carries them through the water for some little distance.
Toads
In some ways toads are like frogs; but you can tell them at once by their rough, dry skins, which are covered with warts like glands. And they crawl over the ground, instead of leaping as frogs do. They are very common almost everywhere, and you may often find them hiding under logs or large stones during the daytime.