Radiolarian protozoan. Light micrograph of a radiolarian Heliodiscus sp., a type of shelled protozoan. These single-celled creatures build hard mineral skeletons around themselves as they float in warm seas with other plankton. They extend their protoplasm as pseudopodia (false feet) through pores in the shell to feed on tiny organisms. They can also obtain food through a symbiotic relationship with the yellow-brown zooxanthellae algae within them. Large numbers of silicaceous radiolarian skeletons litter the sea floor; these will become fossilized to form minerals such as flint. Magnification: x70 at 35mm size.