Centrohelid

The centrohelids or centroheliozoa are a group of heliozoan protists, single-celled eukaryotes with stiff radiating arms (known as axopodia) supported by microtubules and bearing extrusomes (known as kinetocysts). Their cells are spherical, ranging from 3 to 150 μm. Unlike other heliozoa, centrohelids lack flagella, have flat ribbon-shaped mitochondrial cristae, and arrange their microtubules in hexagons or triangles. Their microtubule-generating organelle, the centroplast, has a unique shape with a central trilamellar disc surrounded by two hemispherical caps. Some are naked or covered in a mucous coat, but most centrohelids produce cell coverings, namely organic spicules and siliceous scales of various species-specific shapes. Several species form colonies.

Centrohelids are passive predators with a cosmopolitan distribution. They feed on bacteria, other protists, and invertebrate larvae by phagocytosis; they can merge several cells around a larger prey to ingest it. Although they have been studied in aquatic (mostly freshwater) environments, they are more diverse in soil habitats. They include both free-floating and benthic forms, some of which attach to the substrate by a stalk.