Light micrograph of the pennate diatom Pinnularia major, a member of a division of the algae family. Diatoms are microscopic unicellular organisms that occur abundantly in marine & freshwater plankton. They form the basis of the food chain. The body is composed of two halves, which fit together like a tiny pill box. The cell walls contain silica, which is preserved in deposits known as siliceous or diatomaceous earth (ooze). Once hardened into rock it is called diatomite. The high porosity of this rock enables it to be used for filters, fillers & as a mild abrasive. Magnification: X 400 at 35mm size. photographed through microscope fitted with X40 semi- flat field objective combined with X10 wide-