The tadpole has now become fitted to jump and leap on the land or swim by his legs in the water; and, moreover, while these legs have been growing, another change has been taking place. You will notice by careful watching that at first he still gulps in water as he used to do, but he comes more often to the top, and, poising himself so that his mouth is out of the water, gives out a bubble of bad air, draws in some fresh, and goes down again. Why does he do this? Have you any recollection of another fish-like animal which comes up to take in air? Look back at our friends the mud-fishes (p. 34), and read how the Ceratodus fills his air-bladder when he is short of good air in the water. When you have re-read this, you will suspect that the tadpole, too, has something like an air-bladder, which he fills from time to time. And so he has. While his legs are growing a bag has been forming inside at the back of his throat, which afterwards divides into two, and he fills these by shutting his mouth, drawing air in at his nostrils, putting up the back of his tongue to shut it in, and then swallowing it down into the lungs; so that he is now a truly double-breathing animal, using his gills when below water and his lungs when above. Moreover, if you could watch inside his body, you would now see that little by little the blood-vessels going to the gills grow smaller and smaller, and those going to the lungs grow larger and larger; while the fish’s two-chambered heart divides into three chambers, one to receive the blood from the body, another to receive it from the lungs, and one to drive this blood back again through the whole animal. And when at last this change is so complete that all the blood goes to the lungs to be freshened, the gills shrivel up and disappear, and our tadpole is a true air-breathing animal. Notice, though, that he is still cold and clammy, not warm like a mouse or a bird. For his blood still moves slowly, and as he has only three chambers to his heart instead of four, as warm-blooded animals have, the good blood from the lungs and the worn-out blood from his body become mixed each time they come round, so that his breathing work is still of a low kind all his life. And now that he can leap and swim with his legs, his tail is no longer of use to him, and it is gradually sucked in, growing shorter and shorter till it disappears, and the young frog is complete. Thus our backboned animal has succeeded in getting out of the water on to the land, and in doing so he has quite changed his habits. A peaceful vegetarian before, he is now a greedy eater of insects, slugs, and other animals. His horny beak has been pushed off; his lips have stretched back farther and farther, till they now open right back as far as his flat little ear; and he is a gaping, wide-mouthed, leaping frog[55]-- ... “Hoarse minstrel of a strain Aquatic, leaping lover of the rain;”