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Any protozoon moving by means of protoplasmic flow. In their entirety, the ameboid protozoa include naked amebas, those enclosed within a shell or test, as well as more highly developed representatives such as the heliozoians, radiolarians, and foraminiferans. Ameboid movement is accomplished by pseudopods—cellular extensions which channel the flow of protoplasm. Pseudopods take varied forms and help distinguish among the different groups. A lobe-shaped extension or lobopod is perhaps the simplest type of pseudopod. The shapelessness and plasticity of these locomotory organelles impart an asymmetric, continually changing aspect to the organism. Other, more developed, representatives have pseudopodial extensions containing fibrous supporting elements (axopods) or forming an extensive network of anastomosing channels (reticulopods). Though involved in locomotion, these organelles are also functional in phagocytosis—the trapping and ingesting of food organisms (usually bacteria, algae, or other protozoa) or detritus.

Amebas range from small soil organisms, such as Acanthamoeba (20 micrometers), to the large fresh-water forms Amoeba proteus (600 μm; ) and Pelomyxa (1 mm, or more). Some types, such as Amoeba, are uninucleate; others are multinucleate. Reproduction is by mitosis with nuclear division preceding cytoplasmic division to produce two daughters. Multinucleate forms have more unusual patterns of division, since nuclear division is not immediately or necessarily followed by cytoplasmic division. Transformation of the actively feeding ameba into a dormant cyst occurs in many species, particularly those found in soil or as symbionts. The resting stages allow survival over periods of desiccation, food scarcity, or transmission between hosts.

Amebas are found in a variety of habitats, including fresh-water and marine environments, soil, and as symbionts and parasites in body cavities and tissues of vertebrates and invertebrates. Because of their manner of locomotion, amebas typically occur on surfaces, such as the bottom of a pond, on submerged vegetation, or floating debris. In soil, they are a significant component of the microfauna, feeding extensively on bacteria and small fungi. Amebas in marine habitats may be found as planktonic forms adapted for floating at the surface (having oil droplets to increase bouyancy and projections to increase surface area), where they feed upon bacteria, algae, and other protozoa. Several species of amebas may be found in the human intestinal tract as harmless commensals (for example, Entamoeba coli) or as important parasites responsible for amebic dysentery (E. histolytica).

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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