Radiolaria are amoeboid protozoa with many stiff projections supported by microtubules, called axopods, that produce intricate mineral skeletons. They occur as plankton throughout the ocean, and are known as common microfossils from the Cambrian period onwards. In most there is a central capsule, which separates the inner portion of the cell, or endoplasm, from the outer portion, or ectoplasm. Of these, the endoplasm contains the nuclei and most of the organelles, while the ectoplasm contains many vacuoles and lipid droplets which aid in flotation. In some it contains many symbiotic zooxanthellae, or other algae, which provide much of the cell's energy.
There are two main groups of radiolaria, the Polycystinea and Phaeodarea. Both produce siliceous skeletons, but those of the latter are rarely preserved, so most fossils are of the former. A third group, the Acantharea, produces skeletons of strontium sulfate that do not fossilize. Most likely these different groups do not share a common origin, and they are now usually classified separately. Heliozoa are similar to radiolaria, but lack central capsules and only produce simple skeletal elements, such as spines or scales.
