Protozoa - Protozoa

Japanese: 原生動物 - げんせいどうぶつ（英語表記）protozoa

A general term for single-celled organisms that have a eukaryote and show some animalistic tendencies. In recent classifications, they are sometimes considered to be a group of organisms comparable to animals and plants, or a group belonging to the protista group, but they are not systematic and have no established definition. They are located on the border between multicellular organisms and prokaryotes, and are often thought of as simple organisms because they are single-celled, but they are by no means primitive animals. If animals and plants have progressed to a higher level of organization as a result of their tendency to multicellularity, protozoans can be said to be organisms that have been highly adapted to special environments, just like higher organisms, by aiming for diversification of cell organelles. It is said that the first protozoans were sketches of Euglena and Vorticella in 1674 by the Dutch scientist Leeuwenhoek, and since then, about 65,000 species have been known. In the fields of medicine and parasitology, pathogenic species of protozoa are often called protozoa, or in the plural, protozoa.

[Keiichi Ishii]

distribution

They are widely distributed in freshwater, seawater, and soil, and can be found in hot springs with temperatures over 80°C, in snow, and at depths of over 5,000 meters in the ocean. About one-third are parasitic species, and in particular, all sporozoans are parasitic. Free-living species are generally distributed over a wide area, and there is a strong tendency for the same species to be found in different places. On the other hand, the species that appear are greatly influenced by micro-environmental differences. Therefore, they are also used as an indicator of environmental pollution.

[Keiichi Ishii]

form

Individuals usually range in size from 10 to 250 micrometers. They range from 1 micrometer (hemoflagellates) to 5 mm (giant amoebae), and can reach 6 cm (foraminifera) in shelled specimens. Although they are only a single cell, they perform functions comparable to those of an individual animal. Therefore, the cell itself is highly refined and has highly developed adaptive forms. The nucleus is usually spherical, but can also be rod-shaped, spiral, or necklace-shaped, which is a variety of shapes that cannot be compared to multicellular organisms. Many are multinucleated, and giant amoebae have hundreds of nuclei, and ciliates in particular are characterized by having two types of nuclei, large and small, with different functions. Nuclei can be broadly divided into two types: one located in the center of the nucleus (endosome) and many scattered within the nucleus. They undergo mitosis, but there are also types of division not seen in normal cells, such as those in which the nuclear membrane and nucleus do not disappear during the process, or those without centrioles. The cell surface can be broadly divided into two types: those that secrete a sugar coating on the outside of the cell membrane (called a plasmalemma), those that build various stronger shells (cysts, testes, lorica, etc.) or attach foreign bodies such as sand grains from the outside world, and those that form a differentiated structure (pellicle, shell, etc.) just below the cell membrane. Ciliates in particular have a highly developed outer shell called a pellicle, and even have unique structures similar to those of higher animals, such as a cell mouth, pharynx, esophagus, and anus. In the cytoplasm, organelles such as mitochondria, Golgi bodies, endoplasmic reticulum, ribosomes, and lysosomes are found, just like in normal cells, but the mitochondrial crystals are usually ciliated, and there are also known species that have no mitochondria or only one. There are also rare species that lack Golgi bodies, endoplasmic reticulum, and ribosomes. In addition to the general organelles mentioned above, various other organelles are differentiated in the cytoplasm, such as the contractile vacuole for osmotic regulation unique to protozoans, the food vacuole used for feeding and digestion, the drinking tube, the harpoon, the sucker, the air vacuole for regulating specific gravity, the vacuole, the excretory vacuole for containing urea crystals, the skeletal structure for maintaining body shape, and the microtubule bundles and filaments for contraction, and they share the adaptive functions of an individual.

[Keiichi Ishii]

motion

The three main modes of movement in protozoa are amoeboid movement, flagellar movement, and ciliary movement. Amoeboid movement is a method of movement performed by cells that have plasmalemma as their surface structure. Amoeboid movement is characterized by the formation of temporary movement organs called pseudopodia (also called pseudopodia or pseudopods) during movement, rather than a permanent movement organ. Part of the cell body is deformed and protrudes to become pseudopodia, but the mechanism of amoeboid movement is still largely unknown. They are classified as leaf-like, filamentous, net-like, and axolotled pseudopodia, but the distinction between them is not always clear. Pseudopodia are both a movement organ and directly involved in the formation of food vacuoles. Flagella are swimming organs found in amoeboids and flagellates, and are broadly classified into whip-type and wing-type based on their structure. In addition to the basal granule, some species (such as opalina and hyperflagellates) have additional structures such as a substratum and a wave membrane, and some have more than several hundred flagella. In most species, flagella usually arise from the anterior end of the body. Flagella not only aid in movement, but also create vortexes that aid in feeding, and are involved in adhesion and light reception. Cilia have the same basic structure as flagella, and those that arise from ciliate organisms are called cilia. In addition to the body cilia that usually form a row of cilia on the body surface, many species have compound cilia such as the periostomum, undulatory membrane, synmembrane, and pennycule, and spinous hairs, setae, and tentacles in the abdomen. In addition to the three main movements, gliding and vibratory movements are also important in sporozoans, and the contraction movement of the spasmonome in those with sessile stalks is also important.

[Keiichi Ishii]

nutrition

Protozoans are essentially carnivorous or detritivorous heterotrophs that ingest bacteria, algae, protozoa, small animals, and their fragments by endocytosis (phagocytosis and pinocytosis) into food vacuoles and digest them internally, so they are considered animal-like. However, many flagellates have chromatophores and are autotrophic, and in addition to the common photosynthetic pigment chlorophyll a, some species also contain chlorophyll b, c, and phycobilins. However, they are unique in that they lack lycopene, gamma-carotene, and flavoxanthin, which are common carotene-based accessory pigments in higher plants. Many amoebae and ciliates have symbiotic bacteria or algae. In very rare cases, amoebae have also been known to have an external digestive and absorbing mode.

[Keiichi Ishii]

Pathogenesis

There are about 30 species that are pathogenic to humans, including malaria, various types of trypanosomiasis, leishmaniasis, amebic dysentery, amebic meningitis, trichomoniasis, and toxoplasmosis. There are about 250 species of parasitic species that infect livestock and poultry in Japan, and Trypanosoma, Toxoplasma, Monkey Malaria, Babesia, and Nosema are particularly important because they cause zoonotic diseases. Flagellates, ciliates, and various sporozoans that cause fatal damage to aquatic organisms, especially farmed fish, are also well known, but the red tide phenomenon caused by the abnormal proliferation of dinoflagellates, euglenoids, heterochlorides, and chloromonads is particularly damaging to fisheries.

[Keiichi Ishii]

Reproduction

Reproduction is generally by binary fission, with longitudinal division as typified by flagellates and transverse division as typified by ciliates. In both cases, the process of replicating the wide variety of cell organelles, which is unique to protozoans, is extremely complex. Multiple fission is not uncommon in sporozoans, while unequal fission is not uncommon in ciliates. In some cases, the daughter individuals resulting from division do not separate and form colonies, and in some cases, multicellular division of labor is seen within the colony. In many cases, larvae with completely different morphology from the parents are produced, which metamorphose into adults. Sexual reproduction involves various reproductive methods at each stage, from homozygosity to fertilization. Mating can involve the exchange of only the mating nuclei (Paramecium) or the complete fusion of two cells of different sexes (Chlamydomonas), and unusual methods such as autogamy and self-mating are also commonly seen in ciliates. Cellular slime molds and sporozoans, which have separate sexual and asexual stages, exhibit a highly variable life cycle that combines the formation of spores and cysts.

[Keiichi Ishii]

Classification

The latest classification system was announced in 1980 after discussions at a specialist committee of the International Congress of Protozoology, and divides protozoa into seven phyla: Sarcoflagellates, Labyrinthulida, Apicomplexa, Microsporidia, Acetospora, Myxozoa, and Ciliata. The four phyla below Apicomplexa were lumped together under Sporozoa in the previous classification. This system is not only technical and difficult to understand, but also has names of relatively familiar protozoa that are closely related to human life, such as Radiolaria, Sporozoa, Foraminifera, Slime Molds, and Testate Amoebae, either lost or split into several groups, making it inconvenient for current practical use and likely to take many years to become familiar with. Therefore, for convenience and transition, it is convenient to divide protozoa into four groups: Flagellates, Amoebae, Sporozoa, and Ciliata. Originally, there was no established definition of protozoa, and it is a heterogeneous group of different lineages, and the interrelationships between each group are largely unknown. Furthermore, there are some groups, such as naked amoebae and related groups, that are almost unknown taxonomically. In recent years, there has been a trend to separate most phytoflagellates, including slime molds, labyrinthulins, dinoflagellates, and euglenoids, from protozoa, and many changes to the classification system are expected in the future.

[Keiichi Ishii]

“Illustrated Encyclopedia of Protozoa” edited by Shozo Inoki (1981, Kodansha) ▽ “Animal Systematic Taxonomy 1 Protozoa” by Ryozo Yagyu (1962, Nakayama Shoten) ▽ “Microbial Science” by Tomomichi Yanagida et al. (1980, Gakkai Publishing Center)”

Characteristics of Amoeba Proteus
©Shogakukan ">

Characteristics of Amoeba Proteus

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

真核をもち、多少とも動物的傾向を示す単細胞生物の総称。近年の分類では動物や植物に匹敵する生物群、あるいはプロチスタ群に属する一群とされることがあるが、系統的ではなく、確立された定義もない。多細胞生物と原核生物との境界に位置し、単細胞であるため単純な生物と考えられやすいが、けっして原始的な動物ではない。動物や植物が多細胞化指向の結果、高次の組織化の道を進んだとすれば、原生動物は細胞器官の多様化を指向することによって、高等生物と同様に特殊環境に高度に適応できた生物といえよう。オランダのレーウェンフックが1674年にミドリムシやツリガネムシらしいスケッチを残したのが最初といわれ、以後約6万5000種が知られている。なお、原生動物のうち病原性の種を医学や寄生虫学の分野では原虫、複数形で原虫類ということが多い。

［石井圭一］

分布

淡水、海水、土壌中に広く分布し、水温80℃以上の温泉、積雪中、5000メートル以上の深海にまで生息する。約3分の1は寄生種で、とくに胞子虫類はすべて寄生性である。自由生活種は一般に広範囲に分布し、各地に同じ種がみられる傾向が強い。その反面、微小な環境の相違に出現種は大きく左右される。したがって、環境汚染の指標にも利用されている。

［石井圭一］

形態

10～250マイクロメートルの大きさの個体が普通。最小1マイクロメートル（住血鞭毛虫(べんもうちゅう)）から最大5ミリメートル（巨大アメーバ）、有殻のものでは6センチメートル（有孔虫）にも達する。1個の細胞には違いないが、動物1個体に匹敵する機能を発揮する。したがって、細胞自体は高度に精密化し、各種の適応形態が極度に発達している。核は普通、球形であるが、ほかに長棒状、螺旋(らせん)状、ネックレス状など多細胞生物とは比較にならないほど多彩である。多核のものも多く、巨大アメーバは数百個の核をもち、とくに繊毛虫類には機能の異なる大小2種類の核があるのが特徴である。仁は、核の中央に1個ある場合（エンドソーム）と、多数が核内に散在するタイプとに大別できる。有糸分裂を行うが、その過程で核膜や仁が消失せず、あるいは中心小粒のないものなど普通の細胞にみられない分裂形式もある。細胞表面は細胞膜の外側に糖外被を分泌形成する（プラスマレンマという）か、さらに強固な各種の殻（シスト、テスト、ロリカなど）を構築したり、外界の砂粒などの異物を装着するものと、細胞膜直下に分化した構造（ペリクル、甲など）を形成する種とに2大別できる。繊毛虫はとくにペリクルとよばれる外皮が発達し、細胞口、咽頭(いんとう)、食道、肛門(こうもん)など高等動物と相似の特異構造までもっている。細胞質中にはミトコンドリア、ゴルジ体、小胞体、リボゾーム、リソゾームなど一般細胞と同様の小器官がみられるが、ミトコンドリアのクリスタは普通、繊毛型で、ミトコンドリアのない種や1個だけもつものも知られている。ゴルジ体、小胞体やリボゾームの欠如している種もまれにある。以上の一般的細胞器官のほかに、原生動物独特の浸透圧調節のための収縮胞、摂食消化に使われる食胞、飲食管、銛(もり)、吸管、比重調節の気胞、液胞、尿素系の結晶を入れる排出胞、体形維持の骨格構造と収縮のための微小管束や糸筋など、各種の細胞器官が細胞質中に分化し、1個体としての適応機能を分担している。

［石井圭一］

運動

原生動物の三大運動様式がアメーバ運動、鞭毛運動、繊毛運動である。プラスマレンマを表面構造としてもつ細胞が行う移動法で、恒久的な運動器官によらず、運動時に一時的運動器官である仮足(かそく)（擬足(ぎそく)、偽足(ぎそく)ともいう）が形成されることがアメーバ運動の特徴である。細胞体の一部が変形突出して仮足となるが、アメーバ運動の機構はまだ不明の部分が多い。葉状、糸状、網状、有軸仮足などに分類されるが、その区別はかならずしも明確ではない。仮足は運動器官であると同時に食胞形成に直接関与する。鞭毛はアメーバ類や鞭毛虫類にみられる遊泳器官で、その構造から鞭(むち)型と羽型とに大別される。また、基底小粒のほかに副基体や波動膜などの付属構造を伴ったり、数百本以上の鞭毛をもつもの（オパリナ、超鞭毛虫類など）もある。普通、体前端部より起生する種が多い。鞭毛は運動のみでなく、渦流をつくり捕食に役だて、また付着機能や光の受容にも関与する。繊毛は基本的構造は鞭毛と同じで、繊毛虫より起生するものを繊毛といっている。普通、体表面に繊毛列を形成する体繊毛のほかに、とくに周口部では小膜、波動膜、合膜、ペニクルスなど、腹部では棘毛(きょくもう)、剛毛、触毛などの複合繊毛をもつ種も多い。三大運動のほかに、胞子虫類では滑走運動や振動運動、固着性の柄をもつものではスパスモネームの収縮運動も重要である。

［石井圭一］

栄養

本来、原生動物は食肉型または腐食型の従属栄養で、細菌、藻類、原生動物、小形動物、それらの断片などをエンドサイトーシス（食細胞作用と飲細胞作用）により食胞内に取り込み、体内消化をするので動物的とみなされている。しかし、鞭毛虫類には色素体をもち独立栄養のものも多く、共通の光合成色素クロロフィルaのほかに、bやc、フィコビリンも含まれる種がある。しかし、高等植物に普通のカロチン系補助色素であるリコピン、γ(ガンマ)-カロチン、フラボキサンチンを欠くことは独特である。アメーバ類や繊毛虫類では共生細菌や共生藻をもつ例も多い。ごくまれにアメーバ類で体外消化吸収様式も知られている。

［石井圭一］

病原性

ヒトに病原性をもつものは約30種、マラリアを筆頭に各種トリパノソーマ症、リーシュマニア症、アメーバ赤痢、アメーバ性髄膜炎、トリコモナス腟炎(ちつえん)、トキソプラスマ症が代表的。日本の家畜や家禽(かきん)の寄生種は約250種で、とくにトリパノソーマ、トキソプラスマ、サルマラリア、バベシア、ノセマなどは、人獣共通感染症をおこすので重要である。水産生物、とくに養殖魚に致命的な被害を与える鞭毛虫、繊毛虫、各種の胞子虫類も有名であるが、ことに漁業被害が大きいのは、渦鞭毛虫類、ユウグレナ類、ヘテロクロリス類、クロロモナス類などの異常増殖が原因となる赤潮現象である。

［石井圭一］

生殖

生殖は有糸二分裂が一般的で、鞭毛虫を代表とする縦分裂と、繊毛虫の横分裂とがある。両方とも、原生動物特有の多種多様な細胞器官の複製過程は非常に複雑である。胞子虫類では多分裂、繊毛虫類では不等分裂も珍しくない。分裂の結果生じた娘(じょう)個体が分離せずに群体を生じるもの、群体内で多細胞的な分業のみられるものもある。親とまったく形態を異にする幼生を生じ、変態して成体となる例も多い。有性生殖は同型接合から受精に至るまで各段階の生殖法が行われる。接合にも配偶核だけを交換する場合（ゾウリムシ）と、性の異なる2細胞が完全に融合するもの（クラミドモナス）とがあり、自家生殖や自系接合などの変わった方法も繊毛虫類でよくみられる。有性世代と無性世代とが分かれている細胞性粘菌や胞子虫類などでは、胞子やシスト（胞嚢(ほうのう)）形成が組み合わさって変化に富んだ生活史を示している。

［石井圭一］

分類

最新の分類体系は、1980年に国際原生動物学会議の専門部会の討議を経て発表されたもので、原生動物を肉質鞭毛虫類、ラビリンツラ類、アピコンプレックス類、微胞子虫類、アセトスポラ類、ミクソゾア類、繊毛虫類の7門に大別している。アピコンプレックス以下の4門は従来の分類では胞子虫類に一括されていた群である。この大系は専門的でかなり難解であるばかりでなく、原生動物のなかでも人間生活と関連が深くて比較的なじみ深い放散虫類、胞子虫類、有孔虫類、粘菌類、有殻アメーバなどの名称が消失あるいは数群に分割されているため、現在の実用には不便をきたし、なじむのに長年月を必要とすると思われる。したがって、便宜的かつ過渡的には、鞭毛虫類、アメーバ類、胞子虫類、繊毛虫類の4群に大別するのが便利である。もともと原生動物は確立された定義がなく、系統を異にした各グループの雑居群で、各群の相互関係もほとんど不明である。そのうえ、裸アメーバやその近縁群のように分類学的にほとんど未知のものもある。近年、粘菌、ラビリンツラ類、渦鞭毛虫類、ユーグレナ類を含む大部分の植物性鞭毛虫を原生動物から分離する傾向もあり、今後も分類体系に幾多の改変が予想される。

［石井圭一］

『猪木正三編『原生動物図鑑』（1981・講談社）』▽『柳生亮三著『動物系統分類学1　原生動物』（1962・中山書店）』▽『柳田友道他著『微生物科学』（1980・学会出版センター）』

アメーバ・プロテウスの形質

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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