Coloured scanning electron micrograph (SEM) of a radiolarian. Radiolaria are single-celled protozoans that are found in marine plankton. They have a silicate skeleton with pores through which pseudopodia (false feet, not seen) of protoplasm project. As the animal floats in ocean currents the pseudopodia trap food particles on which the radiolarian feeds. The skeletons, or tests, of ancient radiolarians are used in geological dating, including for oil exploration and determination of ancient climates. This specimen is originally from Mount Diablo in California, USA and dates to the Eocene period (34-56 million years ago). Magnification: x750 when printed at 15 centimetres wide.