of movement is not alone sufficient to distinguish plants from animals. Both one-celled plants and animals require oxygen to maintain life, as has been shown by repeated experiments. The protoplasm of which the cell is composed reacts to the same stimuli in both plants and animals. The one distinction that seems to exist between a plant and animal cell is that in the plant cell food is manufactured by means of the chlorophyll contained in it. Animal cells contain no chlorophyll and require organic food.

HABITAT OF PROTOZOA.-Protozoa are found almost everywhere in shallow water, seemingly never at any great depth. They appear to be attracted near to the surface by light and the supply of oxygen. Every fresh-water lake swarms with them, the ocean contains countless myriads of many different forms.

USE AS FOOD.-They are so numerous in lakes, rivers, and the ocean as to form the food for many animals higher in the scale of life. Almost all fish that do not take the hook and that travel in "schools," or companies, migrating from one place to another, live partly on such food. Many feed on slightly larger animals, which in turn eat the Protozoa. Such fish have on each side of the mouth attached to the gills a series of small structures looking like tiny rakes. These are called the gill rakers, and are used by the fish to collect tiny organisms out of the water as it passes over the gills. The whale, the largest of all mammals, strains protozoans and other small animals and plants out of the water by means of hanging plates of whalebone, the slender filaments of which form a sieve from the top to the bottom of the mouth.

SKELETON BUILDING. - Some of the Protozoa build elaborate skeletons. These may be formed outside of the body, being composed of tiny microscopic grains of sand, or other materials. In some forms the skeleton is internal, and may be made of lime which the animals take out of the water. Still other Protozoa construct shells which house them for a time; then, growing larger, they add more chambers to their shell, forming ultimately a covering of great beauty. These shells or skeletons of Protozoa, falling to the sea bottom, cover the ocean floor to a depth of several feet in places.

The Protozoa have also played an important part in rock building. The chalk cliffs of England and other chalk formations are made up to a large extent of the tiny skeletons of Protozoa, called Foraminifera. Some limestone rocks are also composed in large part of such skeletons.

FLAGELLATES.- Some cells show characters which are like both plants and animals. Such are the group of organisms known as flagellates. All flagellates move through the water by means of one or two (rarely more) long threads of protoplasm, or cilia. Some flagellates are provided with

This spot

chlorophyll, while others appear to take in food in the same manner as animal cells. They have a red pigment spot at one end of the cell body. is sensitive to light, hence it has been called an eyespot. A common flagellate is called Euglena. The green color of stagnant pools is due to the presence of enormous numbers of this organism in the water.

RELATION OF PROTOZOA TO DISEASE. The study of the life history and habits of the Protozoa has resulted in the finding of many parasitic forms, and the consequent explanation of some kinds of disease. One parasitic protozoan, like an amoeba, is called Plasmodium malaria. It causes the disease known as malaria. Part of its life is passed within the body of a mosquito (the anopheles), into the stomach of which it passes when the mosquito sucks the blood from a person having malaria. Within the body of the mosquito a complicated part of the life history takes place, which results in a stage of the parasite establishing itself within the glands which secrete the saliva of the mosquito. When the mosquito pierces its human prey a second time, some of the parasites are introduced into the blood along with the saliva. These parasites enter the corpuscles of the blood, increase rapidly in size, and then form spores. The process of spore formation results

Euglena; F', flagellum; N, nucleus; Pv, contractile vacuole.

in the chill of malaria. Later, when the spores almost fill the blood corpuscle, it bursts, and the parasites enter the blood. There they release a poison which causes the fever. The spores may again enter the blood corpuscles and in forty-eight or seventy-two hours repeat the process thus described.

Another group of infusorian parasites are called trypanosomes. One of this family lives in the blood of native African zebras and antelopes; seemingly it does them no harm. But if one of these parasites is transferred by the dreaded tsetse fly to one of the domesticated horses or cattle of the colonist of that region, death of the animal results.

Another fly is believed to carry a specimen of trypanosome to the natives of Central Africa, and to cause "the dreaded and incurable sleeping sick

ness."

This disease carries off more than fifty thousand natives yearly, and many Europeans have succumbed to it.

CLASSIFICATION OF PROTOZOA

The following are the principal classes of Protozoa, examples of which we have seen or read about:

CLASS I. Rhizopoda (Gk. =root footed). Having no fixed form, with pseudopodia. Either naked as Amaba or building limy (Foraminifera) or glasslike skeletons (Radiolaria).

CLASS II. Infusoria (in infusions). Usually active ciliated Protozoa. Examples, Paramecium, Vorticella.

CLASS III. Sporozoa (spore animals). Usually parasitic and non-active. Example, Plasmodium malaria.

REFERENCE BOOKS

FOR THE PUPIL

Davison, Practical Zoology, pp. 178–184. American Book Company.

Herrick, Text-book in General Zoology, Chaps. II, V. American Book Company. Jordan, Kellogg, and Heath. Animal Studies, Chap. III. D. Appleton and Com

pany.

FOR THE TEACHER

Dodge, General Zoology, pages 54-65. American Book Company.
Calkins, G. N., The Protozoa. The Macmillan Company.

Linville and Kelly, General Zoölogy, Chap. XXI. Ginn and Company.

Parker, T. J., Lessons in Elementary Biology. The Macmillan Company.

Sedgwick and Wilson, General Biology. H. Holt and Company.

Wilson, E. B., The Cell in Development and Inheritance. The Macmillan Company.

XIII. METAZOA

Division of Labor. - If we compare the amoeba and the paramocium, we find the latter a more complex organism than the former. An amoeba may take in food through any part of the body; the paramecium has a definite gullet; the amoeba may use any part of the body for locomotion; the paramecium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramecium is more complex, we say that it is a "higher" animal.

As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work and only that work. This has resulted in what is called division of labor. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and

animals, wherever collections

of cells live together to form

an organism, there is division of labor.

One of the simplest of all colonies is a collection of cells called Pandorina. This is a colony of sixteen cells, whether plant or animal is uncertain, which have become joined by living together in a mass of jellylike material secreted by the cells. They move by means of cilia, and, as a result of living together, move faster through the water and thus obtain more food than one alone. Another form where division of labor is begun is seen in the plant (or animal) called Volvox. This is a hollow sphere of cells, the greater number

Colony of volvox: R, reproductive cells;
C, ciliated cells. (After Kny.)

of which are ciliated and obtain food for the colony; a few, however, have no cilia. These are the reproductive cells, which later break away, and give rise to new colonies when the old one dies.

Protozoa and Metazoa. Thus there have come to exist in the animal world two types of life: the Protozoa, or one-celled individuals, and the Metazoa, or many-celled animals.

In the Protozoa the life processes of growth, waste and repair, and reproduction, are carried on by a single cell. In the Metazoa each of these functions is performed by collections of cells. the Metazoa, too, division of labor becomes increasingly more perfect as we ascend the scale of complexity in form and structure toward the highest type of all, man.

Tissues and Organs. As we have seen in plants, this results in a large number of collections of cells in the body, each collection alike in structure and performing the same function. Such a collection of cells we call a tissue.

Frequently several tissues have certain functions to perform in conjunction with one another. The arm or leg of the human body performs movement. To do this, several tissues, as muscles, nerves, and bones, must act together. A collection of tissues performing certain work is called an organ.

Tissues in the Human Body. Every animal body above the protozoan is composed of a certain number of tissues. The cells making up these tissues have certain well-defined characteristics. Let us see what these cells may be, what their structure is, and, in a general way, what function each has in the human body.

(1) Muscle Cells. A large part of our body is made up of muscle. Muscle cells are elongated in shape, and have great contractile power. In man they may be of two kinds, voluntary (under control of the will) and involuntary.

(2) Epithelial Cells. Such cells cover the outside of a body or line the inside of the cavities in the body. The shape of such cells varies from flat plates to little cubes or columns. Some epithelial cells bear cilia.

(3) Connective Tissue Cells. between tissues in the body. ing numerous long processes.

Such cells form the connection They are characterized by possessThey also secrete, as do many