Radiolarians (also radiolaria) are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and because of their rapid turn-over of species, their tests are important diagnostic fossils found from the Cambrian onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.

German biologist Ernst Haeckel produced exquisite (and perhaps somewhat exaggerated)^{[citation needed]} drawings of radiolaria, helping to popularize these protists among Victorian parlor microscopists alongside foraminifera and diatoms.