Rhizopods, or amebas, are the most primitive of the protozoa. They multiply by simplebinary fission and move by means of cytoplasmic organelles called pseudopodia. These projections of the relatively solid ectoplasm are formed by streaming of the inner, more liquid endoplasm. They move the ameba forward and, incidentally, engulf and inter-nalize food sources found in its path. Most amebas, when faced with a hostile environ-ment, can produce a chitinous, external wall that surrounds and protects them. These forms are referred to as cysts and may survive for prolonged periods under conditions that would rapidly destroy the motile trophozoite. The majority of amebas belong to free-living genera. They are widely distributed in nature, being found in literally all bodies of standing fresh water. Few free-living amebas produce human disease, although two genera, Naegleria and Acanthamoeba, have been implicated occasionally as causes of meningoen-cephalitis and keratitis.
Several genera of amebas, including Entamoeba, Endolimax, and Iodamoeba, are ob-ligate parasites of the human alimentary tract and are passed as cysts from host to host by the fecal–oral route. Several are devoid of mitochondria, presumably because of the anaerobic conditions under which they exist in the colon. Only one, Entamoeba histolyt-ica, regularly produces disease; it has been recently subdivided into two morphologicallyidentical but genetically distinct species, an invasive pathogen that retains the species ap-pellation “histolytica” and a commensal organism, now designated E. dispar. The two species can be differentiated by isoenzyme analysis, antibodies to surface antigens, and DNA markers.