Protozoa

The Protozoa are unicellular non photosynthetic protists. The most primitive protozoa appear to be flagelled forms that in many respects resemble representatives of the algae. It seems likely that the ancestors of these protozoa were algae that became heterotrophs. The nutritional requirements of such organism are met by organic compounds. Adaptation to a heterophic mode of life was sometimes accompanied by loss of chloroplast, and algae thus gave rise to the closely related protozoa. Similar events have been observed in the laboratory as either mutation or physiologic adaptation has given rise to colorless descendants of algal cells. More on Microbial Organisms can be learned from the article linked.

From flagellated protozoa appear to have evolved the ameboid and the ciliated types; intermediate forms are known that have flagella at one stage in the life cycle and pseudopodia the characteristic of the ameba at another stage. A fourth major group of protozoa consists of the sporozoons, parasites with complex life cycles that include a resting or spore stage.

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