ing in swamps with the use of oil, by draining the swamps, or by the introduction of fish which eat the mosquito larvæ has been proved from our experiences along the Panama Canal, in the Philippines, in Cuba, and in New Orleans. Many other diseases of man are probably caused by parasitic protozoans. Dysentery of one kind appears to be caused by the presence of an amoebalike animal in the digestive tract. pox, rabies, and possibly other diseases may be caused by the action of these little animals. Another group of protozoan 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. b a Small How to tell the common mosquito (culex), a, from the malarial mosquito (anopheles), b, when at rest. Note the position of body and legs. an Another fly carries a specimen of trypanosome to the natives of Central Africa, which causes "the dreaded and incurable sleeping sickness." This disease carries off more than fifty thousand natives yearly, and many Europeans have succumbed to it. Its ravages are now largely confined to area near the large Central African lakes and the Upper Nile, for the fly which carries the disease flies near water, seldom going more than 150 feet from the banks of streams or lakes. The British government is now trying to control the disease in Uganda by moving all the villages at least two miles from the lakes and rivers. Why? In this country many fatal diseases of cattle, as tick," or Texas fever, among cattle are probably caused by protozoans. 66 Skeleton Building. Some of the Protozoa build elaborate skeletons. These may be formed outside of the body, being composed of tiny micro scopic 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 beds of Kansas and other chalk formations are made up to a large exSome lime Skeleton of a radiolarian. Highly 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 nonactive. Exam ple, Plasmodium malaria. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Davison, The Human Body and Health, Chap. XXIV. American Book Company. ADVANCED Calkins, G. N., The Protozoa. The Macmillan Company. Linville and Kelly, General Zoology, Chap. XXI. Ginn and Company. Parker, T. J., Lessons in Elementary Biology. The Macmillan Company. Seaman, L. S., "The Sleeping Sickness," Outlook, Jan. 15, 1910. Wilson, E. B., The Cell in Development and Inheritance. The Macmillan Company. XVI. THE METAZOA — DIVISION OF LABOR Problem XXVI. An introductory study of many-celled animals. (Laboratory Manual, Prob. XXVI.) (a) Development. (b) Sponges. (c) The hydra. (d) Development of tissues and organs. (e) Common functions. Reproduction in Simple Plants. Although there are very many plants and animals so small and so simple as to be composed of but a single cell, by far the greater part of the animal and plant world is made up of individuals which are collections of cells living together. In a simple plant like the pond scum, a string or filament of cells is formed by a single cell dividing crosswise, the two cells formed give rise each to two more, and eventually a long thread of cells results. Such growth of cells is asexual. In some instances, however, a single cell was formed by the union of two cells, one from each of the adjoining filaments of the plant. Around this cell eventually a hard coat was formed, and the spore, as it was called, was thus protected from unfavorable changes in the surroundings. Later, when conditions became favorable for its germination, the spore might form a new filament of pond scum. In the seed plants, too, we found a little plant within the seed which, under favorable conditions, might give rise, through the rapid multiplication of the cells forming it, to a new plant. But the plant within the seed first arose from two cells, one of which, called a sperm, came from a pollen grain, the other of which, the egg, was found within the embryo sac of the ovary. Reproduction in Simple Animals. In many-celled animals, as well as many-celled plants, the new animal is formed by the union of a sperm and an egg cell. A common bath sponge, an earthworm, a fish, or a dog,- each and all of them begin life in precisely the same way. Animals which are thus composed of many cells are known as the Metazoa, as distinguished from the Protozoa, which are made of but a single cell. Sexual Development of a Simple Animal. — In a many-celled animal the life history begins with a single cell, the fertilized egg. This cell, as we remember, has been formed by the union of two other cells, a tiny (usually motile) cell, the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed; later this ball sinks Stages in the segmentation of an egg, showing the formation of the gastrula. in on one side, and a double-walled cup of cells, now called a gastrula, results. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg. In the sponge the gastrula, which swims by means of cilia, soon settles down, a skeleton is formed, other changes take place, and the sponge begins life as an animal attached to some support on the water. The early stages of life, when an animal is unlike the adult, are known as larval stages; the animal at this time being called a larva. The young sponge consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ectoderm; the inner layer, developed from the inner layer of the gastrula, the endoderm. A middle almost structureless layer, called the mesoderm, is also found. In higher animals this layer gives rise to muscles and parts of other internal structures. The Structure of a Sponge. The simplest kind of a sponge has the form of an urn, attached at the lower end. A common sponge living in Long Island Sound is a tiny urn-shaped animal less than an inch in length. It has a skeleton made up of very tiny spicules of lime, of different shapes. Cut lengthwise, such an animal is seen to be hollow, its body wall being pierced with many tiny pores or holes. The bath sponge, the skeleton of which is made up of fibers of horn, or a variety known as the finger sponge, shows the pores even better than the smaller limy sponge. In a A horny fiber sponge: IP, the incurrent pores; O, osculum. Notice that this sponge is made up of apparently several individuals. One fourth natural size. bath sponge, however, we probably have a colony of sponges living together. Each sponge has a large number of pores opening into a central cavity, which in turn opens by a larger hole, called the osculum, to the surrounding water. A microscopic examination shows the pores of the sponge to be lined on the inside with cells having a collar of living matter surrounding a single long cilium or flagellum. The flagella, I lashing in one direction, set up a current of water toward the large inner cavity. This current bears food particles, tiny plants and animals, which are seized. Diagram of a simple sponge : I, inhalant openings; O, exhalant opening or osculum. and digested by the collared cells, these cells probably passing on the food to the other cells of the body. The jellylike middle layer of the body is composed of cells which secrete lime to form the spicules and the reproductive cells, eggs, and sperms. |