Amoeba

Amoeba, any of a group of unicellular organisms characterized by their locomotive method of extending cytoplasm outward to form pseudopodia (false feet). The amoeboid group includes hundreds of different organisms, ranging in size from about .25 to 2.5 mm (about 0.0098 to 0.098 in). Amoebas are considered the most primitive animals and are classified in the kingdom Protista. All amoeboid organisms have thin cell membranes, a semirigid layer of ectoplasm, a granular, jellylike endoplasm, and an oval nucleus. Some species live on aquatic plants and some in moist ground; others are parasitic in animals.

Amoebas also use pseudopodia for feeding. Chemical stimuli from smaller organisms, the amoeba's food, induce the formation of pseudopodia, pairs of which envelop the organism, at the same time forming a cavity, or vacuole. A digestive enzyme secreted into the cavity breaks down this food into soluble chemical substances that then diffuse from the cavity into the cytoplasm. Undigested food and wastes are excreted through the ectoplasm, which also absorbs oxygen from the surrounding water and eliminates carbon dioxide, a by-product of metabolism, in a form of respiration. After a period of growth, the amoeba reproduces by splitting into two equal parts.

At least six forms of amoeba are parasitic in humans. Most important of these is Entamoeba histolytica, which causes amebiasis and dysentery. The diseases often occur in epidemics when raw sewage contaminates water supplies or when soil is fertilized with untreated human wastes.