all up—like a boy who has been eating green apples. Isn't it queer that the stomach of the crawfish in the creek, and of the cow in the pasture, should be so much alike? This just goes to show again that you can't judge by outside appearances alone.
The blood of the crawfish circulates very much as the blood of the earthworm does. He has two long tubes for carrying it. One of these tubes runs along his back, the other along the underside of him.
The tube at the top is a vein; the one at the bottom an artery. The vein carries the blood to his heart; the artery carries blood away from his heart. This, you know, is just what your veins and arteries do for you. You can feel the blood beating in one of your arteries by holding the thumb of one hand on the wrist of the other.
As it is much more dangerous to cut an artery than a vein, your arteries are better protected than your veins. For instance, there are veins on the back of your hand, which is always bumping into things, but you have an artery on the inside of your arm and wrist where you seldom get hurt. Mr. Crawfish seems to know he must be more careful of his artery than of his vein. Look at the picture and see where he puts his artery.
Mr. Crawfish seems to be a little careless about the way in which he carries his heart. You see, he has it away up on his back, between his shoulder blades, as it were. But, then, in changing and shifting parts in animals, Mother Nature seems to be a good deal as your mama is with the Spring house cleaning; she can't get everything into the right place at once. But Mr. Crawfish has made the four hearts that he had when he was an angle worm into one heart, and that's a very great improvement.
Now here is a curious thing; the earthworm has four hearts, the crawfish has only one. You have only one; but just look at a picture of a human heart (See Heart, Vol. II, page 853) and see how many parts it has. Four? Yes, just four!
Running along the underside of the earthworm you will notice a little white cord. It is like a thread with knots in it. This is his nervous system; his telegraph line. And the knots are the stations. In the crawfish and his family, there are two of these knotted cords running side by side, and joined together, at the points where the knots are. As Mr. Crawfish thinks mostly about eating and fighting he uses his nerves mostly to run his eating and fighting machines. So we find these little white telegraph wires running around his gullet.
