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Heliozoa Haeckel 1866, stat. n. Margulis 1974, emend. Cavalier-Smith 2012

Diagnosis: Phagotrophic and non-photosynthetic protists with one large spherical or near spherical centrosome at the centre of the cell and numerous radiating axopodia. Axopodia always with extrusomes that are either kinetocysts or tiny dense ellipsoids.

Description: Centrohelids, also called true Heliozoa, are ubiquitous, unicellular amoeboid organisms with slender uniformly radiating axopodia by which they feed. These axopodia are straight, not branching, and possess numerous granules, called kinetocysts, which are used for trapping prey. Each axopodium has a central axoneme with numerous microtubules arrayed in hexagons or triangles. These arrangements can be very different, suggesting that centrohelids are a polyphyletic group. All centrohelids have flat mitochondrial cristae. Most centrohelids are surrounded by a coat or layer of silica scales, which are well characterized by electron microscopy. The morphology and structure of these scales are used to identify most taxa.

Different type of scales

Morphology and ultra-structure of a centrohelid: AX = Axoneme; CV = Contractile Vacuole;  ER = Endoplasmic Reticulum; FV = Food Vacuole; G = Golgi Apparatus; KI = Kinetocyst; M = Mitochondria; MC = MTOC: Microtubule-organizing center; N = Nucleus; PS = Plate-scale; SS = Spine-scale.
Acanthocystis sp., SEM; the colors are artificial – courtesy of © eye of science (www.eyeofscience.com)
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Spiculophrys

Genus Spiculophrys Zlatogursky, 2016 Diagnosis: Centrohelids lacking silica scales but with numerous thin, pointed organic (without any traces of silica) scales tapering towards acute apices. 18S

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Acanthocystis drakena

From Zlatogursky, 2016 Acanthocystis drakena Zlatogursky, 2016 Diagnosis: Cells are 21.1-30.5 µm (ca. 26.7) in diameter. Cell is covered with oval plate-scales having a margin

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Choanocystis symna

From Zlatogursky, 2014 Choanocystis symna Zlatogursky, 2014 Diagnosis: Cell body ca. 6.7 µm in diameter. Axopodia three–five times longer than a cell diameter. Plate-scales dumbbell-shaped

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Acanthocystis costata

From Zlatogursky, 2014 Acanthocystis costata Zlatogursky, 2014 Diagnosis: Cell body ca 9.5 µm in diameter. Axopodia 2–3 times longer than cell diameter. Length of spine-scales

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Kinetocyst

Left: ultra-structure of a kinetocyst of Raphidiophrys contractilis); right: two kinetocysts in an axopodium. Kinetocysts In centrohelid heliozoa, extrusomes are called kinetocysts, and are present

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Yogsothoth carteri

Yogsothoth carteri, after Shishkin and Zlatogursky, 2018 Yogsothoth carteri Shishkin and Zlatogursky, 2018 Diagnosis: Individual cells in a colony have a diameter of 9.7-15.2 µm;

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