to press against it, and soon the plant or animal or organic particle becomes sunken in the protoplasm of the formless body and entirely inclosed in it (Fig. 3). The absorbed particle soon wholly or partly disappears. This is the manner in which the Amaba eats. It has no mouth or

stomach. Any part of its body mass can take in and digest food. The viscous, membraneless body simply flows about the food and absorbs it. Such of the food particles as can not be digested are thrust out of the body.

The Amoeba breathes. Though we can not readily observe this act of respiration, it is true that the Amaba takes into its body through any part of its surface oxygen from the air which is mixed with water, and it gives off from any part of its body carbonic-acid gas. Although the Amaba has no lungs or gills or other special organs of respiration, it breathes in oxygen and gives out carbonic-acid gas, which is just what the horse does with its elaborately developed organs of respiration.

If the Amaba, in moving slowly about, comes into contact with a sand grain or other foreign particle not suitable for food, the soft body slowly recoils and flows for the movement is really a flowing of the thickly fluid protoplasm -so as to leave the sand grain at one side. The Amoeba feels. It shows the effects of stimulation. Its movements can be changed, stopped, or induced by mechanical or chemical stimuli or by changes in temperature.

Amaba is irritable; it possesses irritability, which is sensation in its simplest degree.

If food is abundant the Amaba soon increases in size. The bulk of its body is bound to increase if new substance

FIG. 4.-Amaba polypodia in six successive stages of division. The dark, whitemargined spot in the interior is the nucleus.-After F. E. SCHULZE.

is constantly assimilated and added to it. The Amaba grows. But there seem to be some fixed limits to the extent of this increase in size. No Amaba becomes large. A remarkable phenomenon always occurs to prevent this.

An Ameba which has grown for some time contracts all its finger-like processes, and its body becomes constricted. This constriction or fissure increases inward, so that the body is soon divided fairly in two (Fig. 4). The body, being an animal cell, possesses a nucleus imbedded in the body protoplasm or cytoplasm. When the body begins to divide, the nucleus begins to divide also, and becomes entirely divided before the fission of the cytoplasm is complete. There are now two Amæbæ, each half the size of the original one; each, indeed, being actually one half of the original one. This splitting of the body of the Amaba, which is called fission, is the process of reproduction. The original Amoeba is the parent; the two halves of the parent are the young. Each of the young possesses all of the characteristics and powers of the parent; each can move, eat, feel, grow, and reproduce by fission. It is very evident that this is so, for any part of the body or the whole body was used in performing these functions, and the young are simply two parts of the parent's body. But if there be any doubt about the matter, observation of the behavior of the young or new Amaba will soon remove it. Each puts out pseudopods, moves, ingests food particles, avoids sand grains, contracts if the water is heated, grows, and finally divides in two.

5. Paramecium.-Another protozoan which is common in stagnant pools and can be readily obtained and observed is Paramecium (Fig. 5). The body of the Paramœcium is much larger than that of the Amaba, being nearly one fourth of a millimeter in length, and is of fixed shape. It is elongate, elliptical, and flattened, and when examined under the microscope seems to be a very complexly formed little mass. The body of the Paramecium is indeed less primitive than that of the Amaba, and yet it is still but a single cell. The protoplasm of the body is very soft within and dense on the outside, and it is covered externally by a thin membrane. The body is covered with short fine hairs or cilia,

which are fine processes of the dense protoplasm of the surface. There is on one side an oblique shallow groove that leads to a small, funnel-shaped depression in the body

which serves as a primitive sort of mouth or opening for the ingress of food. The Paramecium swims about in the water by vibrating the cilia which cover the body, and brings food to the mouth opening by producing tiny currents in the water by means of the cilia in the oblique groove. The food, which consists of other living Protozoa, is taken into the body mass only through the funnel-shaped opening, and that part of it which is undigested is thrust out always through a particular part of the body surface. (The taking in and ejecting of foreign particles can be seen by putting a little powdered carmine in the water.) Within the body there are two nuclei and two socalled pulsating vacuoles. These pulsating vacuoles (Amaba has one) seem. to aid in discharging waste products from the body. When the Paramacium touches some foreign substance or is otherwise irritated it swims away, and it shoots out from the surface of its body some fine long threads which when at rest are probably coiled up in little sacs on the surface of the body. When the Paramæcium has taken in enough food and grown so that it has reached the limit of its size, it divides transversely into halves as the Amoeba does. Both nuclei divide first, and then the cytoplasm constricts and divides (Fig. 6). Thus two new Paramacia are formed. One of them has to develop a new mouth opening and groove, so that there is in

FIG. 5.-Paramecium aurelia (after VERWORN). At each end there is a contractile vacuole, and in the center is one of the nuclei.

the case of the reproduction of Paramecium the beginnings of developmental changes during the course of the growth of the young. The young Amabæ have only to add substance to their bodies, to grow larger, in order to be exactly like their parent.

The new Paramacia attain full size and then divide, each into two. And so on for many generations. But it has been discovered that this simplest kind of reproduction can not go on indefinitely. After a number of generations the Paramacia, instead of simply dividing in two, come together in pairs, and a part of one of the nuclei of each member of a pair passes into the body of and fuses with a part

FIG. 6.-Paramecium putorinum dividing. The two nuclei become very elongate before dividing. After BÜTSCHLI.

FIG. 7.-Paramecium caudatum; two individuals separating after conjugation.

of one of the nuclei of the other member of the pair. In the meantime the second nucleus in each Paramecium has broken up into small pieces and disappeared. The new nucleus composed of parts of the nuclei from two animals divides, giving each animal two nuclei just as it had before this extraordinary process, which is called conjugation, began (Fig. 7). Each Paramecium, with its nuclei composed of parts of the nuclei from two distinct individuals,