bottom, the chalky deposits which are found in several parts of the ocean. Our common chalk is composed mainly of the remains of these small shells. Another extensive marine group, the Radiolaria, consisting of several

thousand species, form supporting skeletons, commonly of silica, which are remarkable for the beauty of their radially symmetrical structure. The Foraminifera and the Radiolaria live upon the smaller organisms which they catch with their fine pseudopodia, and they serve in turn as food for higher animals.

The group of protozoans known as the Infusoria are very different in appearance and mode of life from the forms we have been considering. They possess a well defined contour with a permanently differentiated ectoplasm, generally furnished with cilia, which serve as organs of locomotion and frequently also as means of securing food. A typical representative of the Infusoria is the common Paramecium, which is a familiar object to every one who has taken a laboratory course in biology. This organism owes its popularity to the fact that it can be obtained in abundance by making an infusion of hay, or other vegetable material, in water from some pond or stream. After a few days, when the bacteria become abundant, the Paramoecia multiply rapidly and are soon found in large numbers. The elongated body of Paramecium is uniformly covered with short cilia whose oblique beating causes the organism to swim through the water in a spiral path. There is a broad, oblique depression on one side of the body, leading posteriorly to a narrow gullet, which terminates in a small enlargement called the crop. The bacteria and other fine particles which serve as food are swept down the gullet by

ciliary currents and collect in the crop, where at intervals the contents become pinched off by a contraction of the surrounding protoplasm. In this way a mass of food material, together with a small amount of water, is forced into the body, forming a food vacuole. The food vacuoles are carried about by a slow current

cil.

FIG. 27-Paramecium caudatum: cil, cilia; cv, contractile vacuoles; f, food vacuoles; g, gullet with crop at end; n, macronucleus with the micronucleus, n', beside it.

of the semi-fluid endoplasm, and their contents become digested and absorbed during their course. The undigested residue is

nk

m

forced out of the body through a region of the ectoplasm lying behind the mouth. There are two contractile vacuoles which function much as in Amoeba.

Paramecium, like the Infusoria in general, has two kinds of nuclei. Near the middle of the body is the large oval nucleus, called the meganucleus, with a much smaller micronucleus lying close beside it. During fission-for Paramœcium reproduces exclusively by fission-both these nuclei elongate, become constricted in the middle, and divide, the nuclear division being followed by a transverse fission of the whole body. The meganucleus divides amitotically, while the micronucleus undergoes a modified process of mitosis, breaking up into a large number of chromatin threads, or chromosomes, which become pulled apart toward the two poles. Divisions may occur in Paramecium as frequently as once in twentyfour hours, and they may be continued for a great many generations-perhaps indefinitely. At times, however, fission is inter

FIG. 28-Paramacium aurelia di

viding. The macronucleus, m, and

the two micronu

clei, nk, nk', are

in division; o, o',

mouth.

rupted by a form of sexual reproduction, or conjugation. Individuals become attached by their oral surfaces and swim about in pairs. In the meantime, changes take place in the nuclei, which will be described in a later chapter. Each individual receives nuclear material from the other, and after this exchange the organisms separate and then undergo a further series of divisions.

The Infusoria constitute a large group, represented by both marine and fresh-water forms, and also by parasites within the bodies of higher animals. Some of the Infusoria attain a high degree of structural organization which rivals in complexity that of many of the multicellular animals ranking much higher in the scale of life.

The Protozoa belonging to the group of Flagellata, or Mastigophora, are characterized by the possession of flagella, or threadlike organs resembling a whip lash. The flagella are usually one or two, rarely more, in number, and it is by the movements of these organs that the flagellates swim through the water. The group of flagellate organisms is a sort of synthetic group, containing representatives of both plant and animal kingdoms. We may consider these primitive forms as a sort of basic group, from which both the higher plants and the higher animals took their origin.

Of the plantlike flagellates, one of the best known is the common Hæmatococcus, or Spherella pluvialis, whose presence gives sometimes a greenish and sometimes a reddish color to the puddles of water in which it often occurs. In the active condition, Hæmatococcus is oval in shape with a central mass of protoplasm separated from the outer cellulose wall by a space, which is crossed by several transparent protoplasmic strands. At one end there is a protoplasmic eminence, which gives rise to a pair of flagella that protrude through the cell wall. Near the base of these flagella is a reddish body, called an eye-spot, which is supposed to be sensitive to light. There is a round nucleus near the center, and the cytoplasm is colored green by chlorophyll, or red by hæmatochrome, which is only a modifica

tion of chlorophyll. Within the cytoplasm are several plastids which are concerned with the formation of starch.

Hæmatococcus is able to utilize as food only those substances which can diffuse through its cellulose wall. It employs the same mineral substances that are used by the higher plants, and it is similarly dependent upon sunlight, with the aid of its chlorophyll, for building up these materials into living tissue. Its method of nutrition is holophytic, or plantlike, in contrast with that of Amoeba, which is called holozoic. In its ordinary method of reproduction, Hæmatococcus divides within its cell wall into two, and then into four, smaller individuals, which break out

FIG. 29-Hæmatococcus (Sphærella) pluvialis: A, free-swimming stage; B, division within the cell to form four smaller individuals; C, Hamatococcus bütschlüi forming small gametes; D, a free motile spore; E, the conjugation of two gametes; F, G, zygotes formed as the result of conjugation. (After Blochmann.)

of the old encasement and lead a free-swimming life. At intervals, several divisions may rapidly succeed one another within the old cell wall, and give rise to thirty-two or sixty-four small flagellated spores, which leave the old cyst through a rupture in its wall. These small spores are supposed to meet and fuse together, the nuclei combining as well as the cytoplasm. This process of conjugation, or sexual reproduction, is followed by a period of growth and encystment, in which the flagella disappear and the whole organism assumes a spherical form. Encystment also occurs without conjugation. In the encysted state the organism may resist prolonged drought. One may keep the dried cysts for a long time and obtain new active forms at will by placing the cysts in rain water exposed to sunlight.

On account of its green coloring matter, cellulose wall, and

mode of nutrition, Hæmatococcus is considered to be a typical plant. Many other flagellates are quite devoid of chlorophyll and take in solid food. These forms are typical animals. Many flagellates live upon decaying organic substances, which they imbibe through the cell wall. Such forms are called saprophytes. Their mode of nutrition is similar to that of the higher fungi, which are undoubtedly plants, although they have no chlorophyll.

n

FIG. 30-Euglena viridis, a flagellate which com

A considerable number of flagellates are unfortunately parasites. Among the most important of these are the trypanosomes, which infest the blood of vertebrate animals, causing in some cases severe diseases, such as surra and dourine in horses. One species of this group, Trypanosoma gambiense, is responsible for the sleeping sickness which is widespread among the natives of Africa. This disease, as its name implies, is characterized by symptoms of extreme drowsiness in its later stages, and it is almost uniformly fatal. It is estimated that in the Uganda district alone, it carried off 100,000 natives in four years. The fact has been established that the sleeping sickness may be conveyed by the bites of the tsetse fly, but it has not been found feasible to exterminate this insect, and as treatment for the disease has not been thoroughly applied, little has been done to check the woeful slaughter of black humanity that has gone on in Africa for so many years.

Jo

FIG. 31-Trypanosomes.

bines plant characteris

and animal

tics: cv, contractile vacu

oles; e, eye spot; m,mouth; nucleus.

n,

One of the noteworthy traits of the flagellates is their tendency to form colonies. The individuals which are produced by fission may remain together, instead of

separating in the usual manner, and thus build up colonies which assume a variety of forms. Sometimes they consist