Light micrograph of the marine diatom Actino- ptychus ondulatus. 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 and freshwater food chains. The character- istic 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 and bottom of a box. The frustule is often decorated with rows of tiny holes, known as striae, which are arranged either radially (as here) or on the 2 sides of a central axis.