A large group of marine sarcodinids (protists), characterized by having a shell with a perforated, membraneous capsule containing the endoplasm, and a siliceous or strontium sulphate skeleton consisting of a lattice shape of variable morphology made up of spicules, bars, and spines. Radiolarians live mainly in surface waters and the earliest forms are Cambrian in age. They are used in the biostratigraphic correlation of oceanic sediments, particularly where calcareous microfossils have been dissolved.