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[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.
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.
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.
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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.
It is polyphyletic, and it is not a universally recognized classification. It places great significance upon method of locomotion in generating the taxonomy. It can be described either as: kingdom Protista → phylum Sarcomastigophora. or in older classifications as phylum Protozoa → subphylum Sarcomastigophora. [3]
In 1860, British naturalist John Hogg proposed Protoctista (meaning 'first-created beings') as the name for a fourth kingdom of nature (the other kingdoms being Linnaeus' plant, animal and mineral) which comprised all the lower, primitive organisms, including protophyta, protozoa and sponges, at the merging bases of the plant and animal kingdoms.
[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]