<Li> Opisthokonta (in part) <Ul> <Li> Choanozoa </Li> </Ul> </Li> <Ul> <Li> Choanozoa </Li> </Ul> <P> A protist (/ ˈproʊtɪst /) is any eukaryotic organism that has cells with nuclei and is not an animal, plant or fungus . The protists do not form a natural group, or clade, since they have no common characteristic origin, but, like algae or invertebrates, they are often grouped together for convenience . In some systems of biological classification, such as the popular five - kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular - colonial and which form no tissues". </P> <P> Besides their relatively simple levels of organization, protists do not necessarily have much in common . When used, the term "protists" is now considered to mean a paraphyletic assemblage of similar - appearing but diverse taxa (biological groups) that are not related through an exclusive common ancestor, and have different life cycles, trophic levels, modes of locomotion, and cellular structures . In the classification system of Lynn Margulis, the term protist is reserved for microscopic organisms, while the more inclusive term Protoctista is applied to a biological kingdom which includes certain large multicellular eukaryotes, such as kelp, red algae and slime molds . Others use the term protist more broadly, to encompass both microbial eukaryotes and macroscopic organisms that do not fit into the other traditional kingdoms . </P>

When is an organism classified as a protist
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