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<unk>  (singular<unk> n or<unk> n, plural<unk>  or<unk> ns) 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,<unk> ns 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<unk>  was erected as a class within the Animalia, with the word<unk> ' 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<unk>  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<unk> n), and holophyletic (containing all of the known descendants of that common ancestor). The taxon<unk> ' fails to meet these standards, and the practices of grouping<unk>  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<unk>  are Amoeba, Paramecium, Euglena and Trypanosoma. Despite awareness that the traditional taxonomic concept of<unk> " 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<unk>  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<unk> : 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<unk> , 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<unk>  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<unk> , as used in this sense.
Protozoa
Bacteria
Microgenia
Virus
Yeast
Amoeba
Fungus
Microorganism