II. Water Babies That Live in a Village
If the amoeba is ever to get up in the world very far, it must stop using all its parts for everything. The first little creature to make a stomach on the inside, you know very well. Or, rather, you know his empty house. A sponge is the skeleton, or bony house, where hundreds and hundreds of little animals once lived. They all live together so they can help float food to each other's mouths. You know a sponge is full of holes. These holes are long, crooked water streets of the bony town. Other sponge colonies live in glass towns. Those are very beautiful.
Bath sponges are a kind of elastic horn. Elastic means that it will stretch, like rubber. When you fill a sponge with water and squeeze it out, you make it do over again, in a way, what it does when every room in it has living baby tenants. The sponge lives by having water flow through its village. But, instead of soaking up the water and squeezing it out, the water is paddled through the little streets, by little hair-like arms.
The sponge is the lowest animal that is made of more than one cell. The sponge has a stomach on the inside and bones on the outside. By living, a great many of them, in one house or village, like those old cliff-dwellers, all the little sponges get along better. They live very close neighbors, all work for each other, buy their food of the same grocer, and pay for it in a lump. Their house is a kind of fort, too. Larger water animals could "gobble up" millions of separate little sponges, but a whole village of them, in a horny house, is too big and tough a bite. It's a great thing to be sociable, to make friends and live in peace with neighbors. That makes life easier and pleasanter for boys and girls, bees and sponges.
The holes in the sponge are little mouths that lead into the village. Inside of it, when the sponge is alive, there are little one-room houses, and in these are packed, side by side, little jelly-like cells with tails. These tails stick out into the water and, like little fishermen, catch smaller plants and animals out of the water, as it passes through the sponge. At the same time these tails, or fishing poles, catch the water as it flows through the channels. The water also carries air, and the sponge gets its oxygen out of it. You see, the sponge like the fish, must breathe under water.
