Victorian microslide of the arrangement of diatoms by W.H. Watson & Sons, London, England. Diatoms are a distinctive group of single-celled algae containing about 10,000 species. They form a large part of the plankton in marine and freshwater food chains. They consist of two valves, made of silica, which fit together like the top & bottom of a box. The exterior of the valves are often highly decorative, with pores, spikes or other types of projections. Magnification: 16x.