Centrohelid

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{{Taxobox | color = khaki | name = Centrohelids | regnum = Protista | classis = Centrohelea | classis_authority = Kühn 1926 | subdivision_ranks = Families | subdivision = Order Centroplasthelida
   Raphidiophryidae
   Acanthocystidae
   Heterophryidae
Order Axoplasthelida
   Gymnosphaeridae }} The centrohelids or centroheliozoa are a large group of heliozoan protists. They include both mobile and sessile forms, found in freshwater and marine environments, especially at some depth. They are unicellular and spherical, usually around 30-80 μm in diameter, and covered with long radial axopods. These are narrow cellular projections supported by microtubules in a characteristic hexagonal-triangular array, which capture food and allow mobile forms to move about.

The axopodial microtubules usually arise from a tripartite granule called the centroplast. In one group, the gymnosphaerids, they arise from a simpler structure called an axoplast. These also have mitochondria with tubular cristae, whereas in other centrohelids the cristae are flat. Because of this, the gymnosphaerids may be treated as a separate group, but the difference in cristae is no longer considered an indication that they are not related.

A few genera are naked, but most have a gelatinous coat holding scales and spines, produced in special deposition vesicles. These may be organic or siliceous, and come in various shapes and sizes. For instance, in Raphidiophrys the coat extends along the bases of the axopods, covering them with curved spicules that give them a pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules and radiate tubular spicules that are only a little shorter than the pseudopods. Other well-known genera include Heterophrys, Actinocystis, and Oxnerella.