# The How and Why Library/Life/Animals-Section X

X. The Crawfish, the Spider and the Fly
If you were giving Mr. Crawfish, Mr. Spider and Mr. Fly their places in the long line of march, wouldn't you put them in the order I have named: first Mr. Crawfish, then Mr. Spider, then Mr. Fly?
I would, judging just from the looks of them. I would put Mr. Spider next to Mr. Crawfish. He certainly looks a great deal more like Mr. Crawfish than Mr. Fly does.
But we would both be wrong; for the spider is farther advanced in the scale of life than either the crawfish or the fly. So we would have to ask Mr. Fly to fall in behind him in the "procession."
Yet we must not forget there are some things in which the spider is more like the crawfish than the fly is. In form it is plain, he is more like the crawfish.
And, in one thing, the oyster is more like the crawfish than either the spider or the fly. In what way? The oyster has lime in his shell, just as the crawfish has, only a great deal more of it; while flies and other insects have no lime in their shells. Or, to put it in another way, Nature stopped using lime when she made the insects, and took it up again when she got to oysters.
Nature is a great artist in form, in color, in music; she never strikes notes that are too near each other.
Strike two notes on the piano that are side by side and see how they sound. They don't sound "right," do they? And if you play too slowly—letting the sound of one note die away entirely before you strike another, you don't get much of a tune. To make a tune, one note must run into another—the sound of one beginning before the other has stopped.
So, as you see, Nature playing her great harmonies of form and color and sound, you will notice these two things: She doesn't make the different orders of things too much alike. And yet the differences are not so great that you lose the connection.
Now, look again at Mr. Crawfish, Mr. Fly and Mr. Spider. There they go in just that order—one, two, three. Mr. Spider, although he looks so much more like Mr. Crawfish than Mr. Fly, doesn't come next to him in the procession. Doesn't it look as if Nature "skipped a note" when she made him?Not only in the forms of things, but in the stuff they are made of, Nature does skip. She puts lime in the shells of the crawfish and the oyster, but leaves it out of the shells of insects, which come between them. Not all insects have shells, as you know, but such of them as have shells for their backs, or shell-like scales for wings, do not have any lime in them.
Why do the crawfish and the oyster have lime in their shells, while the insects haven't? Think of the lives they lead and you will see the answer. If the crawfishes and the lobster didn't wear strong armor, what would happen to them in the fighting lives they lead? And what would happen to poor Mr. Oyster, who can't fight at all, if his shell were not still thicker?
Flies, grasshoppers, butterflies and a lot of other little friends of yours, belong to the great insect family. There are several reasons why they are placed higher up than the crawfish family. For one thing they have three distinct regions of the body. In that respect they are more like human beings than the crawfish family.
Insects also have but six legs. "Do one thing at a time, and do it well," seems to be Nature's motto. So, in comparing the inner and the outer forms of different members of the animal world we see special parts developing all the time to do new things—as in the case of wings in birds; or to do old things better, as we see when we come to comparing stomachs.
The earthworm has a very simple stomach. His inside is almost all stomach; just a simple tube that digests all the way down. In the crawfish we see these "insides" are pretty much all tube. But, instead of having a lot of legs, like the earthworm, the crawfish has fewer legs; and these legs differ from each other, and are used for different purposes. The earthworm's legs are all alike and are all used for one purpose—to help him get along over the ground.
In the insects we see still fewer legs. Insects always have three pair of legs, while the crawfish has four pairs. Still higher up in the scale are the animals with only two pairs. You see how Nature makes fewer and fewer legs as she goes up and up? Finally she makes two of these four legs into wings, and lo, a bird! Or she makes them into arms, and behold, a little boy or girl! So the legs grow fewer in number and more useful. Think how much better it is to have two real good legs and a pair of arms, than to have as many legs as the earthworm or the "thousand legged" worm, and no arms or wings or anything like that—just legs, legs, legs.As the "outsides" of the animal get more parts, the "insides" must get more parts, too, and each of these parts must begin to do a special work. The amoeba hasn't any real stomach at all; or he's all stomach—whichever way you look at it. By the time we get up to the earthworm, we find a special part that does the digesting. In the crawfish we see part of the earthworm's tube enlarged into a three-roomed stomach at one end, while the rest of the tube runs straight through him, just as it does through the earthworm.
The inside of the earthworm is all one room. The crawfish has two rooms. In the insects this inside space begins to be divided into three rooms. In animals higher than insects, these three rooms are divided more sharply. There is one room for the head, one for the lungs and heart parts, one for the digestion of food.
Even in man there are only these three rooms in the body. It is as if Nature said: "There! A three-roomed house is good enough for anybody."
But, beside more rooms, we need more inside "eating tools." The stomach, the liver, the lungs and other organs, are only inside eating tools. They divide the water, air and food up, more and more, and pass it on. It is so important to improve the inside of an animal's house, to keep up with the improvements on the outside, that Nature seems to stop all outside work for awhile to attend to this. It is in the oyster that we first find the most changes of inside parts. And this is why he is placed so high up in the scale of life, although he looks so very simple on the outside.
As we imagined the earthworm to put on armor and become a crawfish, because we could see the earthworm so plainly inside the armor, now let us imagine the crawfish going into an oyster shell to improve his insides. He takes off his many-jointed legs and eating clippers, takes off his stalked eyes, his long finger-like feelers, shrinks into his shell, makes his shell still harder, so that it will not be easy for enemies to get in and disturb him. There, in his shell castle, he makes better the parts he had, and makes new parts that he never had before.
Of course crawfish never do change into oysters. No animals change into each other in that way. It is more as it is in a family of boys. When they are boys they are pretty much alike, have the same plays, go to the same school—do everything pretty much alike. When they grow older and go out into the larger world, one becomes a lawyer, another a carpenter or a farmer, another a motive engineer, and so on. So far as the things they are interested in and can do is concerned, they are now very different. Even in appearance they have changed a good deal, too, because of these differences in their businesses. But they have changed most of all in their minds. So the forms of animals, as well as their parts, change because of the kind of lives they lead. But it makes it more interesting, sometimes, just to play things; and we are playing now, that the crawfish turned into an oyster.
In the next chapter we will open Mr. Oyster's plain old shell and see what Mr. Crawfish was "up to" when he turned hermit, and went into a shell castle to think things over.