AMOEBA, the name given to certain microscopic animals (see PROTOZOA) which creep along and engulf their prey by out thrust and retraction of pseudo podia, or lobose processes of the naked undivided protoplasmic mass of which their bodies are composed (see PSEUDOPODIUM).
These animalcules are found in fresh water and in the sea, and many live on or in the bodies of other animals, both invertebrate and vertebrate ; man,himself may harbour six different species in his alimentary canal. The first account of an amoeba was that by Roesel, who in 1755 gave a graphic description of the flow ing movements and consequent change of shape in the "proteus" as he called it. The same kind of movement is shown by certain cells of the metazoan body; for instance, by developing eggs of sponges and by the white cor puscles of the blood of many ani mals. The :nyxomycetes, some of the flagellates and certain of the sporozoa also have amoeboid stages in their life - Butschli regarded am o e b o i d movement as a physical phenomenon due to alterations in the surface tension, and he showed that "artificial amoebae" consist ing of drops of oil-soap emulsion, creep about actively for several days by pseudopodia. Verworn and others then demonstrated that the type of pseudopodium formed by an amoeba can be changed by increasing the alkalinity of the surrounding medium.
Pelomyxa, which is a giant of the group, measures as much as 3mm. But the majority of amoebae are not visible to the naked eye, and the small forms that develop in the surface film on an organic infusion measure only about •oimm.
Where it comes in contact with the surrounding fluid, the protoplasm of an amoeba is usually clear and comparatively firm: this layer is the "ectoplasm." The interior "endoplasm" is more fluid and very often granular ; the streaming movements that accompany pseudopodium formation can be followed by watching the circulation of these granules.
There is no mouth or anus ; food is taken in and the undigested re mains cast out at any point on the surface. Amoebae living in ponds or in gently running streams feed on algae, diatoms, and other protozoa; the amoebae in organic infusions devour bac teria, and the same is true of the endoparasites, which are prob ably really harmless scavengers.
Entamoeba histolytica, however, which is known to be the causal organism of a form of dysentery in man, devours red blood cor puscles. The amoebae and as sociated flagellates living in damp earth are believed to exert a con siderable influence on the bac terial content and consequent fertility of the upper layers of the soil.
Each simple protoplasmic mass constituting an amoeba contains a nucleus, usually with a central karyosome and peripheral chromatin, and when the full-grown body divides by binary fission, which is the normal mode of multiplication, the nucleus divides also. This nuclear division involves a scrupulous distribution of the chromatin to the two daughter individuals ; there is, in fact, a modified mitosis (q.v.). In certain cases it has been shown that repeated division of nucleus and cytoplasm within a "cyst" gives rise to a new brood. As yet, no sexual phase has been clearly demonstrated in any true amoeba. A constant feature of all fresh-water amoebae is the fluid-filled "contractile vacuole" (q.v.), which opens to the surface by a pore and is often regarded as conveying nitrogenous waste to the exterior ; the absence of this structure in marine and endoparasitic species suggests that it may also be concerned with regulating osmotic pressure.
The delicate bodies of amoebae are liable to rapid desiccation when exposed to air. They can tide over these periods of danger by adopting a spherical form and surrounding themselves with a protective envelope or cyst. Such cysts may be dispersed by air currents or through the agency of insects such as flies, and if they fall into a suitable environment, the envelope ruptures and the contained amoeba emerges.
Most known species of amoeba can now be cultivated in the laboratory, the bacteria-loving forms on special agar media; and recently certain of the parasitic forms, even those like Entamoeba histolytica from a warm-blooded host, have been successfully grown in vitro. (D. L. M.)