Amoebida

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An order of Lobosia without protective coverings (tests). These protozoa range in size from about 4 micrometers to 0.08–0.12 in. (2–3 mm). Pellicles may be thin, as in Amoeba proteus, or thicker and less flexible, as in Thecamoeba verrucosa. Pellicular folds may develop during locomotion, particularly in species with thick pellicles. Both flagellate and ameboid stages occur in certain soil amebas.

The term ameboid movement is rather loose because locomotion of Amoebida varies somewhat from genus to genus. In some cases movement involves protoplasmic flow of the body as a whole, without typical pseudopodia. In A. proteus there may be several ridged indeterminate pseudopodia, into one of which the organism appears to flow in locomotion. In other species determinate pseudopodia never become large enough to direct locomotion. In some cases the form of the pseudopodia may vary in a single species. Certain amebas commonly have a relatively inert posterior mass, the uroid, which may or may not be partially constricted, is sometimes covered with projections, and often contains food vacuoles. Locomotion involving protoplasmic flow depends upon sol-gel reversibility. See also Cell motility.

Amebas, normally phagotrophic, usually contain food vacuoles. Certain species contain crystals of apparently differing chemical nature. Other inclusions are globules of different sizes, mitochondria, and stored food reserves. In addition, bacteria or algae may occur in the cytoplasm, changing the color to a gray or green. Nuclei range in number from one to several hundred, as in Chaos carolinensis. The giant amebas are visible without a microscope.

Those species found in the digestive tract of invertebrates and vertebrates include relatively harmless species and a few pathogens, such as Entamoeba histolytica of humans and E. invadens of reptiles. Entamoeba histolytica causes amebiasis. In primary cases the amebas are localized in the colon. Cases range from mild amebiasis to acute amebic dysentery.

Entamoeba coli, a similar ameba, does not invade human tissues. Also limited to the lumen of the colon are Endolimax nana, Iodamoeba bütschlii, and Dientamoeba fragilis. These four are relatively harmless although sometimes associated with digestive disturbances. Uncooked vegetables from soil fertilized with human feces are a potential source of infection. Standard methods of water purification seem reasonably protective, but it is difficult to control spread of cysts by food handlers. See also Lobosia; Protozoa; Sarcodina; Sarcomastigophora.

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