The How and Why Library/Life/Animals-Section V
V. Starfish, and Sea Urchins That Play With Live Dolls
Animals like the sponge and the sea-anemone rank higher in the scale of life than the amoeba because they are hollow-bodied. The next higher step is taken by the group of animals to which the starfish, the sea urchin and the sea cucumber belong. They may be said to have real stomachs. The sponge and the anemone only have a special place in the body set apart for digesting food.
Of course there are many strange and interesting things about these animals as there are about everything in the great book of nature, when you come to look at it closely. Probably the amoeba doesn't have as much trouble with his stomach as you do sometimes when you eat more than you should; for if the part of himself he happens to use for a stomach today gets out of order, he can use the other side of himself for a stomach tomorrow, and so give today's stomach a rest. That is the best thing a boy can do for his stomach when it gets out of order. Give it a rest. But the sea cucumber can throw away its stomach and grow a new one.
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A sea animal which seizes its food with the long tentacles seen at top of picture. Through these it breathes also. It moves by the tubes or feet seen on the body, which when filled with water act as suckers and drag the animal over the bottom.
The starfish belong to the spiny-bodied group. They are higher than the sponges and animals of that class, not only in having stomachs more like ours, but in other ways. They are the first animals that begin to walk on solid ground. This they do by forcing water into the suckers with which they get their food. When the suckers are made firm and strong by being filled with water taken into the little animal's mouth, they are firm enough to walk with. (Don't you see how in the starfish, Nature is using over again the sucker foot of the anemone, and his water-filled "petals"?)
The starfish has nerves, as we have already learned. These animals are called starfish because a great many of them are shaped like five-pointed stars. The body is in the center, and the rayscorrespond to our legs and arms. The nerves run along each ray. At the tip of each ray is a little dim eye, and a filmy covering that takes the place of an eyelid. Each ray is really a branch of the stomach. Little canals run from the stomach, which is in the center of the starfish's body, out to the end of each ray.
The starfish can move each ray separately, just as you can move each of your legs or each of your fingers. In this way the starfish is able to travel much faster than you would imagine.
Another member of the great hollow-bodied family, to which you, too, belong, is the sea urchin. In the water, these sea urchins look like round pin cushions stuck all over with black pins, with the points on the outside. When you find them on the sea beach, with all their spines gone, you think them a kind of sea shell. These "pins" are to protect the sea urchin against its enemies. They branch out in every direction like the bayonets of soldiers. That spine armor idea was so good, that Mother Nature used it for the porcupine, and for the thistle and the rose. The sea urchin uses these pins to walk with but, although they have between three and four thousand of these pin feet, they get along very slowly. Whenever we make a specialty of anything we can always do it better. It is when animals come to have two feet, as in the case of man or the ostrich—or four feet, as with squirrels—that they can run and climb trees and build houses and do other things that the sea urchin never even dreams of.
The sea urchin lays eggs, and from these eggs come the young urchins. (What other animal have we already found, that lays eggs?) When they are babies they don't look a bit like their parents. And here is another thing that the sea urchin does that you will remember when we come to kangaroos. Some of the sea urchins carry their babies in pouches. They fold their spines over their babies just as you carry a doll in your arms.
Do you remember, among the strange things that happened in "Alice in Wonderland", that there was a cat that faded and faded away, and left nothing but the grin? You have seen a great many cats without a grin, but a grin without the cat really seems improbable. But this—that I am going to tell you—has happened, not once but many times; no doubt millions of times:
An animal has faded away and left nothing but its mouth! This animal is called the sympata, and is a member of the sea cucumber family. Whenever this animal fails to get food for some time, itseems to say to itself: "If I wasn't so big I wouldn't be so hungry;" so, to save expenses, it drops off a piece of its body. If it still cannot find enough to eat, it drops off another piece—and so on until there is nothing left but its mouth. Then, if it gets something to eat, it begins growing again, and so replaces all those parts of itself that it threw away. The spider and the lobster can grow new legs lost in a fight or accident.
That seems very strange until we remember that when we are ill, or for any other reason eat and digest less than we need, we lose flesh, too, though not all in one spot. Then, when we begin to get well we put it on again.
Perhaps our bodies wouldn't know how to do that if this queer little animal hadn't taught nature how! And perhaps we wouldn't have five fingers and five toes if the starfish hadn't counted five first; nor any eyes or nerves if he hadn't found them for us.
And, Mary, when you carry your doll baby in your arms, remember the sea urchin did it first!
Perhaps she taught you how.
At any rate, there she is—Mrs. Sea Urchin, carrying her little urchins in her spiny arms!