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The centrohelids, or Centroheliozoa, are a large group of heliozoan protists, including both mobile and sessile forms found in both freshwater and marine environments, especially at some depth. They are unicellular, spherical in shape and covered with long radial axopods that capture food and allow mobile forms to move about. These are cellular projections supported by microtubules in a hexagonal-triangular array, which arise from a tripartite granule called the centroplast, located at the center of the cell. The gymnosphaerids, formerly included in this group, have a similar organization, but the centrohelids are distinguished from them and most other heliozoans by having flat rather than tubular cristae within the mitochondria.
Centrohelids are usually around 30-80 μm in diameter. A few are naked, but most have a gelatinous coat holding organic or silicaceous scales and spines, produced in special deposition vesicles, which 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 sort of pine-treeish look, and in Raphidiocystis there are both short cup-shaped spicules as well as radiate tubular spicules that are only a little shorter than the pseudopods. Heterophrys and Actinocystis are other well-known genera.
Protista
