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It is probable that almost all metazoans host a variety of symbiotic protozoa. These symbionts are found on body surfaces (including ciliated epithelia) such as the gills of aquatic organisms and the exoskeleton of crustacea, and are also found in the gut lumen and both intra- and intercellularly in various tissues and organs. A comprehensive treatment of symbiotic protozoa (which include causative agents of major medical and veterinary concern such as malarial, trypanosomal, and amoebal diseases) is beyond the scope of this book. However, symbionts are probably descended from free-living forms which subsequently adapted to life in the special habitats constituted by animals, and such evolutionary events are still taking place. Accordingly there is not a sharp delimitation between free-living and symbiotic protozoa. A few species belonging to the genera Naegleria and Acanthamoeba constitute a wellknown example: These organisms are free-living amoebae in freshwater habitats; on rare occasions, individual cells invade humans to cause extremely virulent and lethal infections (e.g., Page, 1976). The whole spectrum from free-living organisms to facultative or obligate (and lethal) parasites of freshwater invertebrates is represented among species of the ciliate genus Tetrahymena (Corliss, 1973; Batson, 1985). Other examples are sessile ciliates which may regularly attach themselves to aquatic invertebrates.
KeywordsCiliated Epithelium Intestinal Protozoan Habitat Niche Amphipod Crustacean Danish Water
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