Light micrograph of two marine diatoms, Navicula sp.. The diatoms are a distinctive group of single-celled algae containing about 10,000 species. They form an important part of the plankton at the base of the marine & freshwater food chains. The characteristic feature of diatoms is their intricately patterned, glass-like cell wall, or frustule. The frustule consists of 2 halves which fit together like the lid & bottom of a box. The frustule is often decorated with rows of tiny holes, known as striae, which are arranged either radially or, as here, on the 2 sides of a central axis. Magnification: x300 at 35mm size.