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In some systems of biological classification, the Protozoa are a diverse group of unicellular eukaryotic organisms. Historically, protozoa were defined as single-celled organisms with animal-like behaviors, such as motility and predation. The group was regarded as the zoological counterpart to the "protophyta", which were considered to be plant-like, as they are capable of photosynthesis. The terms ''protozoa'' and ''protozoans'' are also used informally to designate single-celled, non-photosynthetic protists, such as ciliates, amoebae and flagellates. The term Protozoa was introduced in 1818 for a taxonomic class, but in later classification schemes the group was elevated to higher ranks, including phylum, subkingdom and kingdom. In several classification systems proposed by Thomas Cavalier-Smith and his collaborators since 1981, Protozoa is ranked as a kingdom. The seven-kingdom scheme proposed by Ruggiero et al. in 2015, places seven phyla under Protozoa: Euglenozoa, Amoebozoa, Choanozoa, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. This kingdom does not form a clade, but an evolutionary grade or paraphyletic group, from which the fungi and animals are specifically excluded. The use of Protozoa as a formal taxon has been discouraged by some researchers, mainly because the term, which is formed from the Greek ''protos'' "first" + ''zoia,'' plural of ''zoion'', "animal", misleadingly implies kinship with animals (metazoa) and promotes an arbitrary separation of "animal-like" from "plant-like" organisms. Modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as "primitive animals." For this reason, the terms "protists," "Protista" or "Protoctista" are sometimes preferred for the high-level classification of eukaryotic microbes. In 2005, members of the Society of Protozoologists voted to change the name of that organization to the International Society of Protistologists. ==History and terminology== The word "protozoa" (singular ''protozoon'', or ''protozoan'') was coined in 1818, by zoologist Georg August Goldfuss, as the Greek equivalent of the German "Urthiere," meaning "primitive, or original animals". Goldfuss erected Protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only microbes, but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoa and polychaete worms. In 1848, in light of advancements in cell theory pioneered by Theodore Schwann and Matthias Schleiden, the anatomist and zoologist C.T. von Siebold proposed that the bodies of microbial organisms such as ciliates and amoebae were made up of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all metazoa. At the same time, he raised the group to the level of a phylum containing two broad classes of microbes: Infusoria (mostly ciliates and flagellated algae), and Rhizopoda (amoeboid organisms). The definition of Protozoa as a phylum or sub-kingdom made up of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology"—and the term came into wide use. As a phylum under Animalia, the Protozoa were firmly rooted in the old "two-kingdom" classification of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microbes and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany. Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae ''Euglena'' and ''Dinobryon'' have chloroplasts for photosynthesis, but can also feed on organic matter and are motile. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms, or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae (protophyta), which he combined together under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum. Six years later, Ernst Haeckel also proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues. Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microbes such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, however, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert F. Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi.〔 A variety of multi-kingdom systems were proposed, and Kingdoms Protista and Protoctista became well established in biology texts and curricula. While many taxonomists have abandoned Protozoa as a high-level group, Thomas Cavalier-Smith has retained it as a kingdom in the various classifications he has proposed. As of 2015, Cavalier-Smith's Protozoa excludes several major groups of organisms traditionally placed among the protozoa, including the ciliates, dinoflagellates and foraminifera (all members of the SAR supergroup). In its current form, his kingdom Protozoa is a paraphyletic group which includes a common ancestor and most of its descendents, but excludes two important clades that branch within it: the animals and fungi. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Protozoa」の詳細全文を読む スポンサード リンク
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