ameba or amoeba (both: əmēˈbə) [key], 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. 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.

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