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Protista

   Also found in: Dictionary/thesaurus, Medical, Wikipedia, Hutchinson 0.01 sec.
Protista (prōtĭs`tə) or Protoctista (prō'tŏktĭs`tə), in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms. Protists, which are eukaryotes eukaryote (y
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, have cells that have a membrane-bound nucleus, DNA that is associated with histone histone (hĭs`tōn), any of a class of protein molecules found in the chromosomes of eukaryotic cells .
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 proteins, and organelles (e.g., mitochondria and chloroplasts). A recently proposed system of classification designates the eukaryotes as one of three great groups of life (beside bacteria and archaea) and places the protists within it.

It has been hypothesized that the organelles in protists descend evolutionarily from specialized symbiotic bacteria living within the cells of other bacteria, contributing at least in part to the transition from prokaryotic (bacterial) cells (the earliest form of life on the planet, dating back at least 3.5 billion years) to early eukaryotic cells (the cells that define protists, dating back 1.5 billion years) and the more complex life forms of later plants and animals.

The protists comprise a very diverse group of organisms. They include some algae algae (ăl`jē) [plural of Lat. alga=seaweed], a large and diverse group of primarily aquatic plantlike organisms.
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, the protozoans protozoan (prō'təzō`ən), informal term for the unicellular heterotrophs of the kingdom Protista .
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, and multicellular or multinucleate autotrophs, such as the water molds water mold, common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista .
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. Many have flagella that enable them to move about. Before the advent of modern biochemistry and the electron microscope, these organisms were fit into the plant and animal kingdoms. It is now thought that, although green plants probably evolved from the green algae and animals from some other early forms, most modern protists have followed independent evolutionary lines. There are approximately 60,000 living species of protists.


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The Protista schema is certainly not yet unanimous.
Typically, myxozoan spores consist of just 6 to 12 cells, not enough for taxonomists to promote these organisms from the kingdom Protista to the status of multicellular animals.
 
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