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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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