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Leishmania donovani, (a species of protozoan) in a bone marrow cell

Protozoa (in Greek proto = first and zoa = animals) are one-celled eukaryotes (that is, unicellular microbes whose cells have membrane-bound nuclei) that commonly show characteristics usually associated with animals, mobility and heterotrophy. Protozoans were commonly grouped in the kingdom Protista together with the plant-like algae and fungus-like water molds and slime molds. In 21st-century systematics, however, most of the algae are classified in kingdoms such as Plantae and Chromista; and in such cases the remaining life forms are occasionally classified as a kingdom Protozoa. But the name is misleading, since protozoans are neither Animalia nor Metazoa, with the possible exception of the enigmatic, moldy Myxozoa).

Protozoa have traditionally been divided on the basis of their means of locomotion, although this is no longer believed to represent genuine relationships:

Most protozoa are too small to be seen with the naked eye—most are around 10–50 μm, but forms up to 0.5 mm exist—but can easily be found under a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. Protozoa occupy a range of trophic levels. As predators upon unicellular or filamentous algae, bacteria, and microfungi, protozoa play a role both as herbivores and as consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria population and biomass. As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important. Protozoa such as the malaria parasites, trypanosomes and leishmania are also important as parasites and symbionts of multicellular animals.

Some protozoa have the ability to form a cyst to protect it from harsh conditions, allowing it to survive exposure to extreme temperatures or harmful chemicals or without food, water, or oxygen for a period of time. For parasitic species the cyst will also enable it to survive outside of the host, allowing it to be transferred from one host to another. An individual protozoan is both male and female.

Another name for protozoa is Acrita (R. Owen, 1861).

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