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PART II. ZOOLOGY
A Hay Infusion. The smallest and simplest plants studied were composed of a single cell; the simplest animals are likewise composed of but one cell. Place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room. Certain changes are seen to take place in the contents of the jar; the water after a little gets cloudy and darker in color; a scum appears on the surface, which is made up of bacteria. These bacteria evidently aid in the decay which (as the unpleasant odor from the jar testifies) is taking place. Later, small one-celled animals appear; these multiply with wonderful rapidity, so that in some cases the surface of the water seems to be almost white with active one-celled forms of life. If we ask ourselves where these animals come from, we are forced to the conclusion that they must have been in the water, the air, or the hay. Hay is
dried grass, which may have been cut in a field near a pool con
taining these creatures. When these pools dried up, the wind may have scattered some of these little organisms in the dried mud or dust. Some may exist in a dormant state on the hay, the water serving to awaken them to active life. In the water too there may have been some living cells, plant and animal. In the decaying hay and in water are cell food in abundance, both inorganic and organic. Living cells increase rapidly here because of the favorable conditions under which they exist. This combination of living and dead matter just described is called a hay infusion.
Study of the Paramecium.1 Let us now take up the study of one of these simple one-celled animals found in a hay infusion. For this purpose the compound microscope, slides, coverslips, a little powdered carmine, and an infusion containing paramecia are necessary. Paramœcia usually appear in a hay infusion in three or four days.
The form of the paramecium, or slipper animalcule, as it is sometimes called, is elongated, oval in outline, and somewhat flattened. Notice that 1 Hunter and Valentine, Manual, page 163.
one end is slightly pointed. This is called the anterior end. Do the animals always move at the same rate of speed? Do they ever turn over, or is one side always uppermost? What happens when they meet an obstruction? The locomotion of the paramecium is caused by the movement of a number of tiny threads of protoplasm, the cilia. These cilia lash the water like a multitude of tiny oars. If a little powdered carmine is allowed to run under the cover glass, the currents of water caused by the cilia may easily be
Paramecium. Greatly magnified. From side. F.V., food vacuole; C.V., contractile vacuole; M, mouth; N, nucleus; W V., water vacuole. (After Sedgwick and Wilson.)
Some of the carmine grains may be found later inside the body of the paramecium. Notice carefully the direction taken by the currents of water bearing the carmine grains (or food particles), and try to locate a funnel-like opening. At the bottom of this funnel is the mouth.
You will notice that the particles of carmine (or food materials) are gathered into little balls within the almost transparent protoplasm of the cell. These masses of food seem to be inclosed within a little area, containing fluid, called a vacuole. Other vacuoles are clear, like those seen in the yeast cell. They are the water vacuoles. One or two other larger vacuoles may sometimes be found, these are the contractile vacuoles; their purpose seems to be to pass off waste material from the cell body. This is done by pulsation of the vacuole, which ultimately bursts, passing out fluid waste to the outside. Solid wastes are passed out of the cell in somewhat the same manner. The nucleus of the cell is not visible in living specimens. In a cell that has been stained it has been found to be a double structure, consisting of one large and one small portion. Make a drawing of a paramecium, showing as many of the above-mentioned parts as you can find.
Response to Stimuli. In the paramecium, as in the one-celled plants, the protoplasm composing the cell can do certain things. Protoplasm responds, in both plants and animals, to certain agencies acting upon it, coming from without; these agencies we call stimuli. Such stimuli may be light, differences of temperature, presence of food, electricity, or other factors of its surroundings. Plant and animal cells may react differently to the same stimuli. In general, however, we know that protoplasm is irritable by some of these factors. To severe stimuli, protoplasm usually responds by contracting, another power which it possesses. We know, too, that plant and animal cells take in food and change the food to protoplasm, that they may waste away and repair themselves. Finally, we know that new plant and animal cells are reproduced from the original bit of protoplasm, a single cell.
REPRODUCTION OF PARAMECIUM. - Sometimes a paramecium may be found in the act of dividing by the process known as fission, to form two
Paramecium dividing by fission. Greatly magnified. M, mouth; MAC., macronucleus; MIC., micronucleus. (After Sedgwick and Wilson.)
new cells, each of which contains half of the original cell. This is a method of asexual reproduction.
Frequently another method of reproduction may be observed. This is called conjugation and somewhat resembles the same process in the thallophytes. Two cells of equal size fuse or merge into one, complicated changes take place in the nuclei of the two cells thus united, and after a short period of rest the one cell divides to form two new individuals. These new animals appear to be rejuvenated as a result of conjuga
tion, and may continue to reproduce asexually by fission for a long period of time. Eventually, however, it seems necessary for the cells to conjugate in order to continue their existence. This stage of conjugation we believed in the plants to be a sexual stage. There seems every reason to believe that it is a like stage in the life history of the paramœcium.
Greatly magnified. M, mouth; Mic., micronucleus; Mac., macronucleus; C. V., contractile vacuole. (After Sedgwick and Wilson.)
Amœba. In order to understand more fully the life of a simple bit of protoplasm, let us take up the study of the amaba, a type of the simplest form of life known, either plant or animal. Amoeba may be obtained from the dead leaves in the bottom. of small pools, from the same source in fresh-water aquaria, from the roots of duckweed or other small water plants, or from green algae growing in quiet localities. No sure method of obtaining them can be given. Unlike the plant and animal cells we have examined, the amoeba has no fixed form. Viewed under the compound microscope, it has the appearance of an irregular mass of granular protoplasm. Its form is constantly changing as it moves about. This is due to the pushing out of tiny projections of the protoplasm of the cell, called pseudopodia (false feet). The outer layer of protoplasm is not so granular as the inner part; this outer layer is called ectoplasm, the inside being called endoplasm. In the central part of the cell is the nucleus. This
Amoeba, with pseudopodia (P) extended; EC., ectoplasm; END., endoplasm; the dark area (N) is the nucleus. From photograph loaned by Prof. G. N. Calkins.
important organ is difficult to see except in cells that have been stained.
The locomotion is accomplished, according to Professor Jennings of the University of Pennsylvania, by a kind of rolling motion," the upper and lower surfaces constantly interchanging positions." The pseudopodia are pushed forward in the direction which the animal is to go, the rest of the body following.
Although but a single cell, still the amoeba appears to be aware of the existence of food when food is near at hand. Food may be taken into the
body at any point, the semifluid protoplasm simply rolling over
and ingulfing the food material.
Within the body, as in the
paramecium, the food is inclosed within a fluid space or vacu
ole. The protoplasm has the power to take out such material as it can use to form new protoplasm or give energy. It will then rid itself of any material that it cannot use. Thus it has the power of selective absorption, a character found in the protoplasm of plants previously studied.
How this cell breathes we cannot say; we know it takes up oxygen and gives off carbon dioxide, as do all other living animals.
Waste products formed from the oxidations which take place in the cell are passed out by means of the contractile vacuole.
The amoeba, like other one-celled organisms, reproduces by the process of fission. A single cell divides by splitting into two others, each of which resembles the parent cell except that they are of less bulk. When these become the size of the parent amoeba, they in turn each divide. This is a kind of asexual reproduction.
When conditions unfavorable for life come, the amoeba, like some one-celled plants, encysts itself within a membranous wall. In this condition it may become dried and be blown through the air. Upon return to a favorable environment it begins life again
From the study of the amoebalike organisms which are known to cause malaria and by comparison with the ambæ which live in our ponds and swamps, it seems likely that every amoeba has a complicated life history during which it passes through a sexual stage of existence. Such a stage is seen in the conjugation of the paramecium.
The Cell as a Unit.
In the daily life of a one-celled animal we find the single cell performing all the activities which we shall later find the many-celled animal is able to perform. In the amoeba no definite parts of the cell appear to be set off to perform certain functions; but any part of the cell can take in food, can absorb oxygen, can change the food into protoplasm and excrete the waste material. The single cell is, in fact, an organism.
Skeleton of Radiolarian. Highly magnified. From model at American Museum of Natural History.
One-celled Plants and Animals Compared. In our consideration of the algae we found that the simplest of all plants consists of a single cell. This cell might be fixed in one place, as the common form of pleurococcus, or it might move about by means of cilia (as seen in the motile stage of pleurococcus and in many other single cells considered to be plants). While single-celled animals are usually free-swimming, nevertheless some (especially parasitic protozoa) do not move about. So the power