Russell-Hunter, W. D. Department of Biology, Syracuse University, Syracuse, New York.
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The kingdom comprising all single-celled forms of living organisms in both the five-kingdom and six-kingdom systems of classification. Kingdom Protista encompasses both Protozoa and Protophyta, allowing considerable integration in the classification of both these animallike and plantlike organisms, all of whose living functions as individuals are carried out within a single cell membrane. Among the kingdoms of cellular organisms, this definition can be used to distinguish the Protista from the Metazoa (sometimes named Animalia) for many-celled animals, or from the Fungi and from the Metaphyta (or Plantae) for many-celled green plants. This system simplifies the lower-level classification (orders and superfamilies) where closely allied forms of flagellate protistans may possess chlorophyll and, therefore, be plantlike in their metabolism, or lack chlorophyll and feed as animals. However, there are other difficulties, not only of semantics but also of fundamental biology, involved in attempting diagnostic separation of the Protista from the other major groups of living organisms. See also: Metazoa
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