This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
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.