(2) How can it be protected from things which hurt it? These two ideas of nature have been worked out in two kinds of bodies. In one of them the soft parts of the body are inclosed in a shell or case of some kind. The soft organs inside are attached to, and protected by, the strong, outer covering. In what animals is this true? You think at once of clams and oysters, do you not? How about starfishes, crayfishes, spiders, beetles, and butterflies?

What is the plan of the body of a fish? As you know very well, the fish has a bony framework, or skeleton, which supports the other organs of the body. The same is true of snakes, alligators, frogs, birds, and all the higher animals, as well as of our own bodies. Some of these animals have outer coverings, such as scales, hair, or feathers, which have developed from the skin, but the real framework is inside. We call those animals with an inside framework vertebrates (věr'tēbratz); while those without the framework we call invertebrates. To which class does the earthworm belong? The jellyfish? fly? The dog? The horse?

The

353. What are Living Cells Like? - Do you remember that after studying about the wonderful organs of a flowering plant like the sweet pea, or the garden bean, we learned that there are plants so simple that instead of having root, stem, leaves, flowers, and seeds, they have just a single cell? See § 309. Usually each cell is so small that it is invisible without a microscope. Did you know that there are one-celled animals, just as there are one-celled plants? The ameba (ă-mē'ba) is one of them. It is a tiny, jelly-like creature found in muddy pools (Fig. 263).

The ameba has no organs for motion, such as feet. When it wishes to move, it extends the wall of the cell out on one side, forming a false foot. The contents of the cell then flow into the false foot, and the creature has moved. The ameba has no mouth; in order to feed itself, it extends two or more false feet about the food, draws away the cell

half of the material of each cell gathers about each part; the two parts separate; and there are now two amebas instead of one.

The higher plants and animals are composed of cells, too, but there are many millions of them. How can such tiny cells make up the plant or animal body? We sometimes compare the cells of our bodies with the cells of a honeycomb, but the cells of the honeycomb are very much larger. Then, too, the honeycomb cells are all alike, and all have the same use, that is, to hold the honey; while in our bodies, and in those of most plants and animals, there are many kinds of cells, of different sizes, shapes, and uses. These cells do their work, and wear out; but they produce new cells to carry on the work, so that the body lives on.

354. What Are Cells Made Of?- How is the material of a cell different from that of a stone, or of a crystal of

salt, or of a honeycomb? The difference is that the cell is living matter; we call it protoplasm. The cell wall is living; so are the nucleus and the other contents of the cell. The living cell differs from a stone in that it can take material out of food and make it a part of itself, and it can carry out reproduction, thus forming other living cells like itself.

How does one of the higher plants or animals grow larger? It does so by the division of its cells, much as in the case of the ameba. You can see that if each new cell grows to the size of the original cell, and there are more cells, the whole body will be larger. However, in the bodies of the higher animals and plants, and in our own bodies, each cell does not do every kind of work, as the one cell of the ameba does. Instead, we have a division of labor; that is, some kinds of cells do one sort of work for the body, and other kinds of cells do different work. Thus the red cells, or corpuscles (kawr'pusls; see § 375), of the blood carry oxygen to all the other cells (see § 46), so that the others can remain where they are. The cells in the stomach and intestines digest food for the other cells (see §§ 368 and 370); while the bone cells build up a strong framework to support the other body cells. What does the group of cells in the lungs do for the other cells?

We call a group of cells of one kind, doing one kind of work, a tissue. Thus we have lung tissue, muscle tissue, bone tissue, and others.

355. What are Bones Like?

Have you ever looked

at the collection of bones left after a chicken, or turkey,

or rabbit, has been eaten, and thought of the many different kinds of bones there are, and that each kind is useful for some particular purpose? There are many different kinds in our own bodies also. We find long bones, short bones, flat bones, round bones, and bones of irregular shape. Why do we find this difference of shape and size? Each bone is especially fitted for the work it is to do, and the support it is to give. The bones of the leg are long to enable us to take long steps. The bones of the head are pieces of irregular shape which fit tightly together, so as to form a hard casing for the brain.

Clean thoroughly the leg bone or wishbone " of a chicken, boil it for a few minutes in water, and soak it for 3 or 4 days in dilute hydrochloric acid (see

§§ 176 and 191). If the action is complete, you will find that the bone has become pliable, and may even be tied in a knot! Why is this? The bone. is composed of branching, interlacing cells of connective tissue (see §§ 195 and 341, and Fig. 264), stiffened with minerals. These minerals make up about of the weight of the bone. The acid acts upon the minerals, so that they dissolve, leaving the connective tissue, which is easily bent. If a bone is burned, the connective tissue is destroyed, and only the mineral matter is left behind; the bone is then very brittle. What quality does the mineral matter give to the bone? The connective tissue?

FIG. 264. Bone cells with canals, or tubes, for the nerves and blood vessels.

Around the bone there is a tough covering of connective material, and the bone tissue itself is very compact and firm. Generally the bones have tubes, or canals," through which the blood circulates; nerves (see § 395) are found there too. Why are the larger bones of flying birds hollow, do you suppose?

356. How Do Bones Grow? Can a very young baby walk? Of course it cannot. We say that its legs are not strong enough to support its body. What are the reasons for this? One is that the baby's muscles are not strong enough to permit it to balance its body properly; but another important reason is that the bones of its legs are not stiff enough. In fact, its legs easily become permanently bent if used too soon.

You know that the bone which supports your nose does not come to the very tip, but that a tough, elastic material takes the place of bone. This is called cartilage (kǎrt'ĭ-lěj). The outer ear contains cartilage, too, instead of bone, and a little growth of thin cartilage exists at the ends of many bones, so as to make a cushion which will prevent the jarring of one bone against another.

The infant's bones are not cartilage, but they are of a material much like it, except that the cells of these soft bones of the infant have the power to produce a stiffening of minerals, and so harden the bones. The bones of an infant "knit," or grow together, easily if broken. The bones of an old person are stiff, but they are also brittle. When they are broken, their cells do not grow easily and join the broken pieces together; so that a fall is often very serious.

The bones of a child need growth materials, such as are found in milk, so that they may harden properly. Review § 191.