Amoeba(redirected from ameboid)
Also found in: Dictionary, Thesaurus, Medical.
ameba or amoeba (both: əmēˈbə), common name for certain one-celled organisms belonging to the phylum Sarcodina of the kingdom Protista. Amebas were previously classified as members of the animal kingdom. Most amebas are very small (from 5 to 20 microns in diameter) and contain a single nucleus. A. proteus averages 0.25 mm in length. Members of the genus Pelomyxa, however, may be well over a millimeter (up to 8 mm) in diameter and may contain hundreds of nuclei. The giant ameba Gromia sphaerica, found on the ocean floor, may reach 1.5 in. (40 mm) in diameter.
Amebas constantly change the shape of their bodies as a result of the phenomenon known as ameboid movement, involving the formation of temporary extensions (pseudopodia, or false feet) of the body. Pseudopodia, used in locomotion and feeding, may be rounded at the tip (lobopodia), pointed (filopodia), branched and fused together (rhizopodia), or somewhat rigid and pointed (axopodia).
Although simple in form, amebas are very successful organisms and are found abundantly in a variety of habitats all over the world. Amebas live in freshwater, the oceans, and in the upper layers of the soil, and many have adapted to a parasitic life on the body surface of aquatic animals or in the internal organs of both aquatic and terrestrial animals. Few animals escape invasion by some type of ameba. Some are harmless, but others are pathogenic and cause serious diseases; e.g., Entamoeba histolytica causes amebic dysentery, which is fatal if untreated, and Naegleria fowleri causes primary amebic meningoencephalitis, which is rare but usually fatal unless treated promptly. The many genera of amebas were given their common name because of their resemblance to the genus Amoeba (order Amoebida), which includes several large, common species of which the freshwater Amoeba proteus is the most familiar.
The term ameba is sometimes also used to refer to other unicellular protists (e.g., slime molds) that have ameboid features such as pseudopodia. Other ameboid protozoans of the phylum Sarcodina include the marine radiolarians, which form silicate skeletons; their freshwater counterparts, the heliozoans; and the shell-bearing foraminiferans.
Digestion and Respiration
the order of the simplest organized protozoa of the class Sarcodina. Most live in fresh waters, a few in the soil; there are parasitic forms. They are usually of microscopic dimensions up to 50 μm, but there are also “giants” such as Pelomyxa, which grows to 2–3 mm. Amoebas have no constant body shape; their cytoplasmic body forms extrusions or pseudopodia which aid in motion and food gathering. They feed on bacteria, minute algae, and protozoa. The amoeba engulfs the food particle and from its cytoplasm the amoeba secretes digestive juices and forms a food vacuole within which the food is dissolved and is incorporated into the cytoplasm. The excretion of water (osmoregulation) and metabolic products occurs through the contractory vacuoles, which gradually accumulate waste matter and discharge it at the surface.
Amoebas ordinarily have one nucleus, but Pelomyxa is multinucleate. The majority of amoebas multiply asexually, dividing in two. The division of the body is preceded by mitosis in the nucleus. A sexual process is known only in very rare cases.
Upon subjection to unfavorable conditions—for instance, insufficient food, cold, or drying of the water body—the amoeba’s body becomes round and the surface of its cytoplasm becomes a dense protective layer; a cyst forms that is resistant to unfavorable effects of the dormant stage. Freshwater amoebas can serve as a water pollution index. Parasitic amoebas live in the intestines of various animals and man. Among these the most harmful is the dysentery amoeba (Entamoeba histolytica), which produces a severe form of amoebic dysentery in man. Infection occurs via cysts, which remain viable outside the human body.
REFERENCESDogel’, V. A., Iu. I. Polianskii, and E. M. Kheisin. Obshchaia protozoologiia. Moscow-Leningrad, 1962.
Seravin, L. N. Dvigatel’nye sistemy prosteishikh. Leningrad, 1967.
A. A. STRELKOV