Ameba - Ameba (English spelling)

Japanese: アメーバ - あめーば（英語表記）ameba

A general term for the amoeboid amoebae, which belong to the class Thallusimus of the phylum Sarcomastigellata of protozoa. They are also called amoebas. In a broader sense, they can refer to testate amoebae, axolotls, or cells that perform amoeboid movement in other animals and plants.

[Hideki Horikami]

form

The body surface is wrapped in a cell membrane called plasmalemma, and is characterized by the formation of temporary pseudopodia as locomotor organs. The morphology of the pseudopodia is also an important classification characteristic. The inside of the cell is differentiated into a flowing sol endoplasm and a cylindrical gel-like exoplasm. The endoplasm contains nuclei, food vacuoles, mitochondria, contractile vacuoles, and various granules. The ectoplasm becomes sol at the tail and flows to the front of the body as endoplasm, generating and protruding plasmalemma at the tips of the pseudopodia. At the same time, it piles up on the ectoplasm on both sides of the tip, gelling and causing additional growth of pseudopodia. This cell movement based on sol-gel conversion at the front and rear ends of the body is called amoeboid movement.

[Hideki Horikami]

Ecology

They live widely in freshwater, seawater and soil, and some are parasitic and pathogenic. Some species produce flagella at one stage of their life cycle, while others form cysts when environmental conditions change. They reproduce by mitosis, usually binary fission. They do not have a permanent mouth, and digest food intracellularly. Their feeding methods include phagocytosis, in which bacteria, other protozoa, algae, etc. are wrapped in pseudopodia and taken into the body by food vacuoles, and pinocytosis, in which liquid nutrients are taken in by invaginating the plasmalemma.

[Hideki Horikami]

Main types

Amoeba proteus is the most well-known freshwater amoeba, with a body length of 300-500 micrometers. When moving forward, one or more pseudopods emerge from the front half of the body, and there is one disk-shaped nucleus. There is one contractile vacuole at the rear of the body. It is carnivorous. Pelomyxa palustris is the largest amoeba, with some reaching a body length of 5 mm. It is sausage-shaped with a single pseudopod, and is multinucleated, but does not have mitochondria, Golgi apparatus, or contractile vacuoles. It lives symbiotically with rod-shaped bacteria in its cytoplasm, and often lives in nutrient-rich black mud. It is omnivorous, and its body is usually dark brown. It is semi-anaerobic. Naegleria gruberi is a small soil amoeba with a single pseudopod type, and during one stage of its life cycle, it produces two flagella and swims. It forms cysts. Thecamoeba verrucosa is a representative species of amoeba with a thick outer shell, with a body length of 100 to 180 micrometers. When moving, its external shape is an inverted triangle. There are several wrinkled ridges on the dorsal side along the anterior-posterior axis of the body. It is found in freshwater. Flabellula citata is flattened and fan-shaped, with a body width greater than its length. The granules are concentrated at the rear of the body, so the front half is occupied by a transparent layer. It is found in the sea.

[Hideki Horikami]

Characteristics of Amoeba Proteus
©Shogakukan ">

Characteristics of Amoeba Proteus

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

普通には原生動物の肉質鞭毛虫(べんもうちゅう)門葉状仮足綱に属する無殻アメーバの総称。アミーバともよぶ。広義には有殻アメーバや有軸仮足類、あるいは他の動植物のアメーバ運動を行う細胞をさすこともある。

［堀上英紀］

形態

体表はプラスマレンマとよばれる細胞膜で包まれ、運動器官として一時的な仮足を形成するのが特徴。また、仮足の形態が分類の標徴として重視される。細胞内は流動するゾル性の内質と、それを円筒状に取り巻くゲル性の外質とに分化している。内質中には核、食胞、ミトコンドリア、収縮胞や種々の顆粒(かりゅう)が含まれる。外質は尾部でゾル化し内質となって体の前部へ流れ、仮足先端のプラスマレンマを新生して突出させると同時に、先端両側の外質に積み重なり、ゲル化して仮足の付加成長を生ずる。この体の前端部と後端部でのゾル・ゲル転換に基づく細胞の移動がアメーバ運動である。

［堀上英紀］

生態

淡水、海水、土壌中に広く生息し、寄生性で病原性を有するものもある。生活環の一時期に鞭毛を生ずる種や環境条件の変化でシスト（包嚢(ほうのう)）をつくる種もある。増殖は有糸分裂で、普通、二分裂である。恒久的な口はなく、細胞内消化のため摂食法には、細菌、ほかの原生動物、藻類などを仮足で包み込み、食胞をつくって体内に取り入れる食作用と、液性栄養物をプラスマレンマを陥入させて取り込む飲作用とがある。

［堀上英紀］

おもな種類

アメーバ・プロテウスAmoeba proteusは、淡水産のもっとも有名な種で、体長300～500マイクロメートル。前進中は体の前半部より1本から数本の仮足を出し、核は円盤状で1個。体の後部に1個の収縮胞をもつ。肉食性。ペロミクサ・パルストリスPelomyxa palustrisは、アメーバ類中最大の種で、体長5ミリメートルに達するものもある。単一仮足のソーセージ状で、多核であるがミトコンドリア、ゴルジ体、収縮胞をもたない。桿菌(かんきん)を細胞質中に共生させ、富栄養の黒色泥中によく生息する。雑食性で、体は普通暗褐色。半嫌気性。ネグレリア・グルベリNaegleria gruberiは、単一仮足型の小形土壌アメーバで、生活環の一時期に2本の鞭毛を生じて遊泳する。シストを形成。テカメーバ・ウエルコーサThecamoeba verrucosaは、外皮の厚いアメーバの代表種で、体長100～180マイクロメートル。移動中の外形は逆三角形。背側に体の前後軸に沿って数本のしわ状隆起がある。淡水産。フラベルラ・キタタFlabellula citataは、扁平(へんぺい)な扇形で、体長よりも体幅が大きい。顆粒(かりゅう)は体後部に集まるので、前半部は透明層で占められる。海産。

［堀上英紀］

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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