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 are similar to radiolaria, but they are distinguished from them by lacking central capsules and other complex skeletal elements, although some produce simple scales and spines. They may be found in both fresh water and marine environments.

Originally the heliozoa were treated together as a formal class Heliozoa or Heliozoea, but the various orders show notable differences, and are no longer believed to be related. Instead, heliozoa is regarded as a descriptive term applying to various different lines of protists. These include the following:

• Actinophryids and some other axodines
• Centrohelids
• Desmothoracids
• Gymnosphaerids
• Dimorphids
• Sticholonche

Several nucleariids were once considered heliozoa, but they do not have microtubule-supported axopods and so are now considered filose amoeboids instead.