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The kingdom-level classification of life is still widely employed as a useful way of grouping organisms, notwithstanding some problems with this approach: Kingdoms such as Protozoa represent grades rather than clades, and so are rejected by phylogenetic classification systems.
[1] [b] In the 21st century, the classification shifted toward a two-kingdom system of protists: Chromista (containing the chromalveolate, rhizarian and hacrobian groups) and Protozoa (containing excavates and all protists more closely related to animals and fungi). [2] The following groups contain protists.
This classification remained widespread in the 19th and early 20th century, [7] and even became elevated to a variety of higher ranks, including phylum, subkingdom, kingdom, and then sometimes included within the similarly paraphyletic Protoctista or Protista.
The initial targets of Cavalier-Smith's classification, the protozoa were classified as members of the animal kingdom, [12] and many algae were regarded as part of the plant kingdom. With growing awareness that the animals and plants embraced unrelated taxa, the use of the two kingdom system was rejected by specialists.
While there has been considerable debate on the classification of protozoa caused by their sheer diversity, in one system there are currently seven phyla recognized under the kingdom Protozoa: Euglenozoa, Amoebozoa, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa.
The five-kingdom model remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that protists are a paraphyletic group from which animals, fungi and plants evolved, and the three-domain system (Bacteria, Archaea, Eukarya) became prevalent. [181]
[7] [9] In traditional classification schemes, Amoebozoa is usually ranked as a phylum within either the kingdom Protista [10] or the kingdom Protozoa. [11] [12] In the classification favored by the International Society of Protistologists, it is retained as an unranked "supergroup" within Eukaryota. [7]
Type of protist Movement mechanism Description Example Other examples Motile Flagellates: A flagellum (Latin for whip) is a lash-like appendage that protrudes from the cell body of some protists (as well as some bacteria).