The 71st plate from German biologist Ernst Haeckel's Kunstformen der Natur, showing radiolarians of the order Stephoidea. Radiolarians form intricate mineral skeletons, usually with a central capsule dividing the cell into inner and outer portions. Radiolarians are found as zooplankton throughout the ocean and are important diagnostic fossils, found from the Cambrian period onwards.Image credit: Ernst Haeckel