Light micrograph of a single shell of a radiolarian. Radiolaria are single-celled protozoans that are found in marine plankton. They have a silicate skeleton with pores through which pseudopodia of protoplasm project to trap food particles. As zooplankton in the ocean their skeletal remains form siliceous ooze on the seabed. Magnification: x220 when printed at 10 centimetres across.