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Protozoa or Cornelius protozoans (from Greek proton proton "first" and ζῷα zoa "animals"; singular protozoon; (the word "protozoan" is originally an adjective, used as a noun) are microorganisms classified as unicellular eukaryotes. They play a key role in maintaining the balance of the ecosystem. 
While there is no exact definition of the term "protozoan", most scientists use the word to refer to a unicellular heterotrophic protist, such as an amoeba or a ciliate.
Protozoa usually range from 10–50 μm, but can grow up to 1 mm, and are easily seen under a microscope. They move around with whip-like tails called flagella. They formerly fell under the protista family. Over 30,000 different types have been found. Protozoa exist throughout aqueous environments and soil, occupying a range of trophic levels. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoa play a role as both herbivores and consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria populations and biomass. Protozoa may absorb food via their cell membranes, some, e.g. amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food. All protozoa digest their food in stomach-like compartments called vacuoles.
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 (Plasmodium spp.), trypanosomes and leishmania are also important as parasites and symbionts of multicellular animals.
Some protozoa have life stages alternating between proliferative stages (e.g. trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures and harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of the host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek, tropho=to nourish), they actively feed and grow. The process by which the protozoa takes its cyst form is called encystation, while the process of transforming back into trophozoite is called excystation.
Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, while some use a combination, (eg. Coccidia). An individual protozoon is hermaphroditic.
Protozoa were previously often grouped in the kingdom of Protista, In the 21st-century systematics, protozoa, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. However, protozoa are not Metazoa (with the possible exception of the enigmatic Myxozoa).[How to reference and link to summary or text]
Protozoa have traditionally been divided on the basis of their means of locomotion, although this character is no longer believed to represent genuine relationships:
- Flagellates (e.g. Giardia lamblia)
- Amoeboids (e.g. Entamoeba histolytica)
- Sporozoans (e.g. Plasmodium knowlesi)
- Ciliates (e.g. Balantidium coli)
-  Protozoa, defined at Microbe World. 2006 American Society for chemistry. Retrieved June 15, 2008.
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