Diatoms. Computer-enhanced engraving of assorted species of diatom. Diatoms are a group of single-celled algae. The two main morphological types of diatom are represented here: pennate (rod-shaped) and centric (rounded). A characteristic feature of diatoms is their intricately patterned, glass-like cell wall, or 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 floating plankton in the marine and freshwater food chains. Engraving taken from Ernst Haeckel's famous treatise: 'Art Forms of Nature' (1904).