The Structure of Paramœcium.—The cell body is almost transparent, and consists of semifluid protoplasm which has a granular grayish appearance under the microscope. This protoplasm appears to be bounded by a very delicate membrane through which project numerous delicate threads of protoplasm called cilia. (These are usually invisible under the microscope).
The locomotion of the paramœcium is caused by the movement of these cilia, which lash the water like a multitude of tiny oars. The cilia also send particles of food into a funnel-like opening, the gullet, on one side of the cell. Once inside the cell body, the particles of food materials are gathered into little balls within the almost transparent protoplasm. These masses of food seem to be inclosed within a little area containing fluid, called a vacuole. Other vacuoles appear to be clear; these are spaces in which food has been digested. One or two larger vacuoles may 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 fluid waste to the outside. Solid wastes are passed out of the cell in somewhat the same manner. No breathing organs are seen, because osmosis of oxygen and carbon dioxide may take place anywhere through the cell membrane. The nucleus of the cell is not easily 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, called, respectively, the macronucleus and the micronucleus.
Paramœcium dividing by fission. M, mouth; MAC., macronucleus; MIC., micronucleus. (After Sedgwick and Wilson.)
Reproduction of Paramœcium.—Sometimes a paramœcium may be found in the act of dividing by the process known as fission, to form two new cells, each of which contains half of the original cell. This is a method of asexual reproduction. The original cell may thus form in succession many hundreds of cells in every respect like the original parent cell.
Amœba, with pseudopodia (P.) extended; EC, ectoplasm; END, endoplasm; the dark area (N.) is the nucleus. (From a photograph loaned by Professor G. N. Calkins.)
Amœba.[25]—In order to understand more fully the life of a simple bit of protoplasm, let us take up the study of the amœba, a type of the simplest form of animal life. Unlike the plant and animal cells we have examined, the amœba 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 locomotion is accomplished by a streaming or flowing of the semifluid protoplasm. The pseudopodia are pushed forward in the direction which the animal is to go, the rest of the body following. In the central part of the cell is the nucleus. This important organ is difficult to see except in cells that have been stained.
Although but a single cell, still the amœba appears to be aware of the existence of food when it is near at hand. Food may be taken into the body at any point, the semifluid protoplasm simply rolling over and engulfing the food material. Within the body, as in the paramœcium, the food becomes inclosed within a fluid space or vacuole. The protoplasm has the power to take out such material as it can use to form new protoplasm or give energy. Circulation of food material is accomplished by the constant streaming of the protoplasm within the cell.
