This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
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.
In his head you will find several of these nerve knots grown together. And that's little Mr. Crawfish's little brain.
Mother Nature doesn't "cross bridges" until she comes to them; that is, she takes care of the business of every day without bothering herself too much about what she is going to do a long way ahead. She's not like the little girl who got to dreaming how much money she was going to get for her eggs, and then how, by and by, she was going to sell more eggs, and so finally get enough to buy a silk dress. You know, while she was going along thinking of everything but where she was going, she tripped and fell.
And the eggs—!
Mother Nature always has her mind on her day's work. She says : " Give us this day our daily duty, and the doing of it will keep us happy and get us ready for the next duty. "