Radiolarian (Eucyrtidium ampulla) skeleton. Radiolaria are protozoa, single-celled eukaryotes, typically 0.1mm - 0.2mm across. They form part of the zooplankton in the oceans. The radiolarian shown here belongs to the Polycistina group and so has a silica-based skeleton that preserves well in the fossil record. False-coloured scanning electron micrograph (SEM). Image is x493 life size when printed 10cm across.