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ANIMAL LIFE

CHAPTER I

THE LIFE OF THE SIMPLEST ANIMALS

1. The simplest animals, or Protozoa. The simplest animals are those whose bodies are simplest in structure and which do the things done by all living animals, such as eating, breathing, moving, feeling, and reproducing in the most primitive way. The body of a horse, made up of various organs and tissues, is complexly formed, and the various organs of the body perform the various kinds of work for which they are fitted in a complex way. The simplest animals are all very small, and almost all live in the water; some kinds in fresh water and many kinds in the ocean. Some live in damp sand or moss, and still others are parasites in the bodies of other animals. They are not familiarly known to us; we can not see them with the unaided eye, and yet there are thousands of different kinds of them, and they may be found wherever there is water.

In a glass of water taken from a stagnant pool there is a host of animals. There may be a few water beetles or water bugs swimming violently about, animals half an inch long, with head and eyes and oar-like legs; or there may be a little fish, or some tadpoles and wrigglers. These are evidently not the simplest animals. There will be many very small active animals barely visible to the unaided eyes. These, too, are animals of considerable complexity. But if a single drop of the water be placed

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on a glass slip or in a watch glass and examined with a compound microscope, there will be seen a number of extremely small creatures which swim about in the water-drop by means of fine hairs, or crawl slowly on the surface of the glass. These are among our simplest animals. There are, as already said, many kinds of these "simplest animals, although, perhaps strictly speaking, only one kind can be called simplest. Some of these kinds are spherical in shape, some elliptical or football-shaped, some conical, some flattened. Some have many fine, minute hairs projecting from the surface; some have a few longer, stronger hairs that lash back and forth in the water, and some have no hairs at all. There are many kinds and they differ in size, shape, body covering, manner of movement, and habit of food-getting. And some are truly simpler than others. But all agree in one thing-which is a very important thing—and that is in being composed in the simplest way possible among animals.

2. The animal cell.-The whole body of any one of the simplest animals or Protozoa is composed for the animal's whole lifetime of but a single cell. The bodies of all other animals are composed of many cells. The cell may be called the unit of animal (or plant) structure. The body of a horse is complexly composed of organs and tissues. Each of these organs and tissues is in turn composed of a large number of these structural units called cells. These cells are of great variety in shape and size and general character. The cells which compose muscular tissue are very different from the cells which compose the brain. And both of these kinds of cells are very different from the simple primitive, undifferentiated kind of cell seen in the body of a protozoan, or in the earliest embryonic stages of a many-celled animal.

The animal cell is rarely typically cellular in character -that is, it is rarely in the condition of a tiny sac or box of symmetrical shape. Plant cells are often of this char

acter. The primitive animal cell (Fig. 1) consists of a small mass of a viscid, nearly colorless, substance called protoplasm. This protoplasm is differentiated to form two parts or regions of the cell, an inner denser mass called the nucleus, and an outer, clearer, inclosing mass called the cytoplasm. There may be more than one nucleus in a cell. Sometimes the cell is inclosed by a cell wall which may be simply a tougher outer layer of the cytoplasm, or may be a thin membrane secreted by the protoplasm. In addition to the protoplasm, which is the fundamental and essential cell substance, the cell may contain certain so-called cell products, substances produced by the life processes of the protoplasm. The cell may thus contain water, oils, resin, starch grains, pigment granules, or other substances. These substances are held in the protoplasm as liquid drops or solid particles.

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FIG. 1.-Blood cell of a crab (after HAECKEL). Showing cytoplasm and nucleus (the large, inner, nearly circular spot) and granules of various substances lying in the cytoplasm.

The protoplasm itself of the cell shows an obvious division into parts, so that certain parts of it, especially parts in the nucleus, have received names. The nucleus usually has a thin protoplasmic membrane surrounding it, which is called the nuclear membrane. There appear to be fine threads or rods in the nucleus which are evidently different from the rest of the nuclear protoplasm. These rods are called chromosomes. The cell is, indeed, not so simple as the words "structural unit" might imply, but science has not yet so well analyzed its parts as to warrant the transfer of the name structural unit to any single part of the cell-that is, to any lesser or simpler part of the animal body than the cell as a whole.

The protoplasm, which is the essential substance of the cell and hence of the whole animal body, is a substance

of a very complex chemical and physical constitution. Its chemical structure is so complex that no chemist has yet been able to analyze it, and as the further the attempts at analysis reach the more complex and baffling the substance is found to be, it is not improbable that it may never be analyzed. It is a compound of numerous substances, some of these composing substances being themselves extremely complex. The most important thing we know about the chemical constitution of protoplasm is that there are always present in it certain complex albuminous substances which are never found in inorganic bodies. It is on the presence of these albuminous substances that the power of performing the processes of life depends. Protoplasm is the primitive basic life substance, but it is the presence of these complex albuminous compounds that makes protoplasm the life substance. A student of protoplasm and the fundamental life processes, Dr. Davenport, has said, “Just as the geologist is forced by the facts to assume a vast but not infinite time for earth building, so the biologist has to recognize an almost unlimited complexity in the constitution of the protoplasm." *

*The physical structure of protoplasm has been much studied, but even with the improved microscopes and other instruments necessary for the study of minute structure, naturalists are still very far from understanding the physical constitution of this substance. While the appearance of protoplasm under the microscope is pretty generally agreed on among naturalists, the interpretation of the kind of structure which is indicated by this appearance is not at all well agreed on. Protoplasm appears as a mesh work composed of fine granules suspended in a clearer substance, the spaces of the mesh work being composed of a third still clearer substance. Some naturalists believe, from this appearance, that protoplasm is composed of a clear viscous subtance, in which are imbedded many fine granules of denser substance, and numerous large globules of a clearer, more liquid substance. Other naturalists believe that the fine spots which appear to be granules are simply cross sections of fine threads of dense protoplasm which lie coiled and tangled in the thinner, clearer protoplasm. And, finally,

3. What the primitive cell can do.-The body of one of the minute animals in the water-drop is a single cell. The body is not composed of organs of different parts, as in the body of the horse. There is no heart, no stomach; there are no muscles, no nerves. And yet the protozoan is a living animal as truly as is the horse, and it breathes and eats and moves and feels and produces young as truly as does the horse. It performs all the processes necessary for the life of an animal. The single cell, the single minute speck of protoplasm, has the power of doing, in a very simple and primitive way, all those things which are necessary for life, and which are done in the case of other animals by the various organs of the body.

4. Amoeba. The simple and primitive life of these Protozoa can be best understood by the observation of living individuals. In the slime and sediment at the bottom of stagnant pools lives a certain specially interesting kind of protozoan, the Ameba (Fig. 2). Of all the simplest animals this is as simple or primitive as any. The minute viscous particle of protoplasm which forms its body is irregular in outline, and its outline or shape slowly but constantly changes. It may contract into a tiny ball; it may become almost star-shaped; it may become elongate or flattened; short, blunt, finger-like projections called pseudopods extend from the central body mass, and these projections are constantly changing, slowly pushing out or

others believe that protoplasm exists as a foam work; that it is a viscous liquid containing many fine globules (the granule-appearing spots) of a liquid of different density and numerous larger globules of a liquid of still other density. It is a foam in which the bubbles are not filled with air, but with liquids of different density. This last theory of the structure of protoplasm is the one accepted by a majority of modern naturalists, although the other theories have numerous believers. But just as with what little we know of the chemical constitution of protoplasm, the little we know of its physical structure throws almost no light on the remarkable properties of this fundamental life substance.

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