pellicle (biology)

Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are 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 (originally spelled 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 classifying Protozoa within the context of Animalia was widespread in the 19th and early 20th century, but not universal. By the 1970s, 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 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. 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''.

Despite 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. 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. A 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 Alveolata and Stramenopiles, under 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 descendants of Protozoa, as used in this sense.

Science

The word "protozoa" ''(singular ''protozoon'')'' was coined in 1818 by zoologist Georg August Goldfuss (=Goldfuß), as the Greek equivalent of the German ', meaning "primitive, or original animals" (' ‘proto-’ + ' ‘animal’). Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. From p. 1008: ''"Erste Klasse. Urthiere. Protozoa."'' (First class. Primordial animals. Protozoa.) ote: each column of each page of this journal is numbered; there are two columns per page./ref> Originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoa and polychaete worms. The term ''Protozoa'' is formed from the Greek words (), meaning "first", and (), plural of (), meaning "animal". The use of Protozoa as a formal taxon