A free-living, usually freshwater, protozoan that has a spherical shell and radiating projections: Heliozoa, or sun animalcules, are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface.

These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food, sensation, movement, and attachment. They may be found in both fresh water and marine environments.