The How and Why Library/Life/Animals-Section I
I. The Little Animal that Walks With its Stomach and Eats With its Feet
Don't you like the menagerie part of the circus best? And the "Zoo" in the city park? Wild animals are so strange and interesting. In every pond and creek there are animals that are just as strange. You don't have to buy a ticket to see them, as you do for a circus. But you can see a great many more of them, and all of them a great deal better, if you have a good microscope. Some of these little animals are wonderfully small, as well as wonderfully made.
The lowest forms of animal life, as of vegetable life, live in the water. The very, very smallest animal is just a single cell, too small for you to see without a magnifying glass. The yeast plant, you know, is alike all over, and gets its food by "soaking it in through its skin."
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an amoeba feeding
In the first figure a bit of food lies near it, in the second figure it draws near the food, next it stretches around it and then it swallows it.
The first figure shows how the amoeba changes into a solid mass if touched, and the last two figures show the amoeba dividing so as to make two.
One of these single-celled animals is called the amoeba. That is a Greek word that means "change." The amoeba can change its shape whenever it wants to. If you could put on a pair of wings as easily as the amoeba can make feet, you could do what every boy has wanted to do—fly like a bird. Whenever the amoeba wants to move in a certain direction, little legs push out from that side of its body and draw the rest of the amoeba after it. Whenever it wants to go in another direction it draws in these legs and makes legs on the other side.
But when it wants to eat it doesn't make a mouth, and put food in this mouth, as you might suppose. Whenever it touches the food on which it lives, it simply wraps itself around it like a little boy trying to carry a big watermelon.
It looked very odd to see the hair-like growths of the flower cup close around the bee and make it give up as much of the pollen on its legs as the plant wants, before letting go. It looked as if even flowers could think sometimes. So the little amoeba acts as if it could think, too. It seems to have ideas and tastes just as we have. Not so many ideas, and not so many tastes; but you would hardly expect that of an amoeba, would you? An amoeba is thousands and thousands of times smaller than we are—hardly a hundredth of an inch across its little body. Just think what might happen, if we were as many times brighter than an amoeba as we are times larger!
For see what it does:
If it wraps itself around a piece of food that is too big for it— if it "finds its eyes are bigger than its stomach" as little boys do sometimes—it just unwraps itself from the food and glides away. Sometimes it seems not to like the taste of things, for, having wrapped itself around something, it holds it awhile and then lets go of it again without eating it. Some of the little animals it eats have shells. When it is through with one of these it unwraps itself and drops the shell.
And it seems to have nerves, too. Of course we can't tickle its feet because it hasn't any except those that it makes when it wants to go walking. But if you touch it, or shake it, it pulls all of itself in, making itself into a little round drop of jelly.
You have often noticed how an earth-worm, or fishing-worm, as you call it, will shrink when you touch it. This shows that the earth-worm has "feelings," too. It has something that answersfor nerves. We couldn't get along very well without nerves because it is through them that we know what is going on around us. Nerves are just as necessary to make things go right inside of us. It is by means of the nerves of the eye that we see, the nerves of the ear that we hear, the nerves of the tongue that we taste.
As the cells of plants change into roots or leaves or bark, when leaves or roots or bark are needed, so all the different parts of animals, from the amoeba up to man, have been made to grow by the work they, have to do. The amoeba uses all parts of itself for the same purposes. There is no part that always does the walking—so it has no legs or feet. The legs and feet which the amoeba makes, as it wants them, are called " false feet." It has no stomach that always stays a stomach, because there is no part that it always uses to digest its food.
There is a little animal called the "moneron" which is still lower in the scale of life than the amoeba. For one thing it hasn't any skin—this moneron. Inside and outside it is just the same. The amoeba has a kind of a skin on the outside, and a little hollow place on the inside, which serves both as a heart and lungs, distributing the food and oxygen from the water throughout its body. The oxygen which it needs comes out of the air just as does the oxygen which we need. You know there is air in the water.
Next above the amoeba are little animals called in-fu-sor'ia. These, under the microscope, look like caps or bells, with little hairs all around them. They remind us of the little whips that help to make the spores in the liver-wort, as if to say that plants and animals are related. These infusoria go thrashing around in the water just as the whip-tailed cells of the liver-wort do, using the hairs to swim with, just as the liver-wort cells uses these whips. These little hairs stay little hairs, and are not drawn back and changed into something else, as are the feet of the amoeba.
Still higher up are other little animals that look something like these, but in addition to having these little hairs to swim with, they have mouths that stay mouths all the time. In these little animals part of the hairs are used like oars to swim with, and those around the mouth are used as hands, fanning other little animals into the mouth.
Now don't you believe that, in a pool of standing water, there are just as wonderful animals as in a circus? And don't you see, also, that in the animal kingdom, nature begins spelling out her wonderful story, in little easy words of one syllable, just as you learned to read when you began with the primer:
"It is a c-a-t."
Only, when we read the Book of Nature we can't begin with the cat; she's away up in a higher grade, with the fish and the birds and boys and girls. She has a backbone; and these little animals in the pond menagerie haven't any bones at all!
Your big brother or sister who goes to high school, can tell you more about the amoeba and other simple forms of life. Or, if you have a very fine microscope, he can show them to you. You may find amoeba on the dead leaves in the bottom of pools, or in the home or school aquarium, or on the roots of duck weed and other small water plants. You can also put some hay or straw in a glass jar filled with water, let it stand a few days in a warm room, and get specimens of another kind of one-celled animals. Then you can watch them through the microscope. See Amoeba, page 64; Biology, page 212; Protoplasm, page 1554; Protozoa, page 1554; Infusoria, page 925.