AMICS'BA ( Neo-La t. ; Gk. altotg7j, change, alternation). A microscopic animalcule, classified among the lowest Protozoa, which in habits fresh water or occasionally moist earth. It appears in water under great magnification as a clear, translucent, highly refracting body. It is made up of a substance that does not mix with water, is viscid like glue, and has a specific gravity a little greater than water, namely about 1.015. Under the highest power of the microscope, partieularly after death, the body is seen not to be perfectly homogeneous, but to be made up of films inclosing water-filled spaces. This structure may be imitated by making a fine mixture of oil and potassium carbonate, and letting it stand in water. The potassium carbon ate is hygroscopic, and eventually an emulsion is produced in which the oil stands to the water in the same relation that the protoplasmic films do to the water spaces. The cytoplasm is not all of the same kind. Near the centre is a special ized portion known as the nucleus, the proto plasm outside of 1.1'11 MI is celled cytoplasm. Be tween nucleus and cytoplasm a constant. inter change of material is taking place in the living cell, and the two parts are interdependent.
A living amceba under appropriate cdonditions exhibits a continual movement of the protoplasm. The very structure favors a constant movement, as witness the artificial emulsion, whose outline is constantly changing. In amceba processes (pseudopodia) are thrust out at certain points of the body, and others behind them are re tracted, and thus a change occurs in the centre of the mass and locomotion is effected. Ameba is irritable, i.e., it is affected in a definite man ner by external conditions; it "responds" to. them by moving with reference to them. With out a stimulus there would probably be HO move ment at all. If the stimulus (Tunes upon the or ganism from one side, it may move toward or from that side_ Thus the anucha moves from the point of contact of a needle or from the source of light. or it moves so as to keep in water of a medium temperature. Thus we see that pro
toplasm had the capacity of appreciating external conditions and moving with reference to them. This may be regarded as the beginning of a "psychic life." • The amceba is, by its movements, constantly ex pending energy. This must be renewed by taking in fuel. Also, its plasma is undergoing constant destruction and must be reformed. Food here serves two purposes. (1) It serves as fuel . (2) It forms new protoplasm. But these two rules may be played by the same food-stuff. There is no proper oral aperture, and the food is merely taken into the interior of the body by a process of intussus•eption—any portion of the surface being chosen for this purpose, and acting as an extemporaneous mouth. Sometimes the inges tion of food takes place chiefly at the posterior end of the body. When the particle of food has been received into the body, the aperture by which it was admitted again closes up, and the discharge of solid cxereta is effected in an exact ly similar but reverse manner. Food-stuffs that are ingested become dissolved (digested) and penetrate the plasm films. Here they are burned (oxidized) and carbon dioxide, water, urea, and other substances are produced. These get back into the water spaces and are finally thrown out with the "contractile vacuole." This process is excretion. But in the plasma film, where the food-stuff was burned, there is an increase of temperature. This heat is used in part in chem ical work—in the buiicling up of new living mole cules from food stuffs. Ilere• then, is a great chemical Ial nratoy in the protoplasm.
"The 'contractile vesicles' are cavities within the endosarc, of which ordinarily- only one is present in the same individual, though there may Ile two or more. In position, the contractile vesi cle, or `pulsating vacuole.' as it is often called, is usually placed toward the hinder end of the body, as is also the nucleus." Reproduction takes place by simple division, each amoeba. as it reaches maximum size, split ting in two, as shown in the illustration. See