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. 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.

When first introduced by Georg Goldfuss in 1818, the taxon Protozoa was erected as a class within the Animalia, with the word 'protozoa' meaning "first animals". 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. The approach of classying Protozoa within the context of Animalia was fitting to the 19th and early 20th century perspective. It was not universal. By the 1970's, it became usual to require that all taxa are monophyletic (derive from a common ancestor that would also be regarded as protozoan), and are holophyletic (containing all of the known descendants of that common ancestor). The taxon 'Protozoa' fails to meet these standards; and the practices of grouping protozoa with animals and of treating them as closely related, are no longer justifiable. 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. Some examples of protozoa are Amoeba, Paramecium, Euglena and Trypanosoma.

Over the same period, the spelling and meaning of relvant terms has changed. Initially, an organism in this group was referred to as a 'protozoon'. The plural of this noun was 'protozoa', and it became the name of the taxon (as 'Protozoa'), now capitalized. The term 'protozoan' was used as an adjective. This traditional European-style spelling has been increasingly replaced by a more US-based spelling, in which the singular organism is referred to as 'protozoan', the plural being 'protozoans or protozoa', the formal name remains 'Protozoa', and the noun 'protozoan is also used as an adjective.

Despite awareness that the traditional taxonomic concept of "Protozoa' did not meet contemporary taxonomic standards, some authors continuued to use the name but applying it to differing scopes of organisms. This has caused ambiguity. In a series of classifications by Thomas Cavalier-Smith and collaborators since 1981, the taxon Protozoa was applied to a different circumscription and was ranked as a kingdom. A different scheme presented by Ruggiero et al. in 2015, places eight not closely related phyla within Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. 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 the Alveolata, Stramenopiles, or the polyphyletic Chromista. The Protozoa in this scheme do not form a monophyletic and holophyletic group (clade), but a paraphyletic group or evolutionary grade because it excludes some descendents of Protozoa used in this sense.

Protozoology as a discipline emerged in the 19th century. Many workers still refer to themselves as protozoologists.

 

Note:   The above text is excerpted from the Wikipedia article Protozoa, which has been released under the GNU Free Documentation License.