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 loco-
