A eukaryotic organism which is single-celled and photosynthetic is placed under the kingdom Protista. Usually, protists need not be related to each other and most are completely unique from other members of this kingdom. A common characteristic feature that of protists is that they are eukaryotes, which means they have a “true” nucleus.
Protists are neither plants nor animals, hence are informally classified under their own kingdom. Members under this kingdom do not have a common ancestry nor share a common evolutionary path. Hence, the members are widely varied and are mostly unrelated to each other.
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