Circumscription: The centrohelids, one of several groups previously assigned to the Heliozoa that are now regarded as polyphyletic. Round bodied with stiff radiating arms (axopodia) supported internally by microtubules and with extrusomes that are quite easy to see. Cell surface naked or with organic or siliceous spines and/or scales. Some species with symbiotic algae. No flagellated stages known, may form cysts. Ultrastructural identity: Mitochondria with lamellate flat cristae. Microtubules of radiating axonemes in hexagon with triangle arrangement and arising from a multilamellate microtubule organizing centre, extrusomes with ball-and-cone organization. Radiating system of lacunae near cell surface. Many conventional. dictyosomes dispersed in cytoplasm. Siliceous elements of periplast created within silicon deposition vesicles. Synapomorphy: Platycristate heliozoa with multilamellate microtubule organizing centre faced with hemispherical structures giving rise to axonemes of microtubules arranged in hexagons and triangles. Composition: About 85 species; Family Heterophryidae (Cienkowskya, Oxnerella, Sphaerastrum, Heterophrys), Family Raphidiophryidae (Parasphaerastrum, Polyplacocystis, Raphidiophrys, Raphidocystis,) Family Acanthocystidae (Pseudoraphidocystis, Pseudoraphidiophrys, Pterocystis, Echinocystis, Choanocystis, Acanthocystis).