Diatoms. Light micrograph of assorted species of diatom, a group of single-celled algae. Pennate diatoms (at upper right & left and lower right) are symmetrical about a line. The rest are centric diatoms which are symmetrical about a point. A characteristic feature of diatoms is their intric- ately patterned, glass-like cell wall, the frustule. The frustule consists of two halves which fit together like the lid and bottom of a box. It is often decorated with rows of tiny holes, known as striae. Diatoms form an important part of the plankton in the marine and freshwater food chains. These specimens were found in the North Sea. Magnification: x160 at 4x5 inch size.