like the rounds of a ladder, as you see from the picture. After the worm's food is digested into blood, it oozes out into the rest of his body. The different parts of the worm are bathed in it, and so are made to grow. Most of the earthworm's blood is used in this way; so only a part of it is passed back through the hearts. These hearts do not have so much to do as your heart does, and that is why they are so simple. In the next higher group of animals—those with a shell, like the crawfish—we will find the heart is not so simple.
An earthworm showing the sections or rings of muscle.
You notice the earthworm is made up of sections or rings of muscles, just as your backbone is made up of rings of bone. So he not only hints at the heart and blood vessels of animals higher than himself, but he seems to say:
"See how useful it is to be made up of rings. See how I can bend and turn and twist and get in and out, everywhere. The animals below me cannot do this. After awhile there will come an animal with a backbone, made up of rings. He will crawl on the ground, and be called a serpent. Then will come other animals with stiffer backbones and feet. They will not have a great many hooklike feet as I have. They will have only four good, jointed feet. In the water, also, there will be animals with backbones. Instead of feet they will have paddles or fins. Into the air will come animals with backbones, two feet and two fins, called wings. With these wings they will swim the air.
"Last of all will come the most wonderful animal of all. At first, when he is a baby, he will creep about on his little stomach, just as I do. Then he will go about on four legs for a while—will creep on his hands and feet. Then, when his backbone grows stronger, and he has learned to stand alone, as the fern learned to do long ago, he will begin to walk with two of his legs, and the other two legs, now called arms, will be set free to use in other ways.
"On these arms will be hands, and on these hands five fingers, like the five rays of the starfish and the five petals of a flower. With these five fingers he will grasp, first of all, his food, as the star fish does with his five rays. Then with these hands and fingers he will make boats to go about in the water like fish. At first he will make only play boats, then, as he grows older, big boats, with fins, called paddles. After awhile he will make other boats with paddles or wings for swimming in the air. These the will call flying machines.
