Notably, this approach excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, which were located in other groups such as Alveolata and Stramenopiles, under the polyphyletic Chromista. in 2015, places eight not closely related phyla within Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. In a series of classifications by Thomas Cavalier-Smith and collaborators since 1981, the taxon Protozoa was applied to a restricted circumscription of organisms, and ranked as a kingdom. ĭespite awareness that the traditional taxonomic concept of 'Protozoa' did not meet contemporary taxonomic standards, some authors have continued to use the name, while applying it to differing scopes of organisms. Some examples of protozoa are Amoeba, Paramecium, Euglena and Trypanosoma. The term continues to be used in a loose way to describe single-celled protists (that is, eukaryotes that are not animals, plants, or fungi) that feed by heterotrophy. The taxon 'Protozoa' fails to meet these standards, and the practices of grouping protozoa with animals, and treating them as closely related, are no longer justifiable. By the 1970's, it became usual to require that all taxa be monophyletic (derived from a common ancestor that would also be regarded as protozoan), and holophyletic (containing all of the known descendants of that common ancestor). The approach of classifying Protozoa within the context of Animalia was widespread in the 19th and early 20th century, but not universal. In later classification schemes it was elevated to a variety of higher ranks, including phylum, subkingdom and kingdom, and sometimes included within Protoctista or Protista.

When first introduced by Georg Goldfuss (originally spelled Goldfuß) in 1818, the taxon Protozoa was erected as a class within the Animalia, with the word 'protozoa' meaning 'first animals'. Historically, protozoans were regarded as 'one-celled animals', because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae. Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Clockwise from top left: Blepharisma japonicum, a ciliate Giardia muris, a parasitic flagellate Centropyxis aculeata, a testate (shelled) amoeba Peridinium willei, a dinoflagellate Chaos carolinense, a naked amoebozoan Desmarella moniliformis, a choanoflagellate

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