These may be elongate, with a bilateral plane of symmetry, or they may be round and radially symmetrical. Many diatoms are slightly asymmetrical, though they generally fall into one of these two categories. The skeleton of a diatom, or frustule , is made of very pure silica coated with a layer of organic material. Observe the diatom frustule below at right, in which the two halves have been pushed slightly askew. As is visible in the photographs, both parts of a frustule may be highly perforated.
Pennate diatoms show a long slit, the raphe , along the long axis.
Through the raphe, the living diatom secretes mucilage, with which it may attach to a substrate or move by gliding over the substrate. Within their silica walls, diatoms show a typical level of eukaryote organization . Living diatoms contain several chloroplasts, where photosynthesis takes place. These photographs were provided by UCMP Museum Scientist Dr. Karen Wetmore. For more photographs of diatoms, visit the Algal Microscopy and Image Digitization server at Bowling Green State University.