Calcareous phytoplankton. Coloured scanning electron micrograph (SEM) of a coccolithophore, a small marine algal organism. The coccosphere consists of plates (coccoliths) of calcium carbonate. Each cell has two flagellar structures, which are involved not only in motility, but also in mitosis and formation of the cytoskeleton. These skeletons can make up the major component of a limestone rock that was laid down in the Cretaceous period, between 142 and 65 million years ago. Magnification: x5500 when printed at 10 centimetres wide