Once known as blue-green algae, cyanobacteria are bacteria that are able to use photosynthesis to make foods in a similar way to plants. Oscillatoria willei and other related cyanobacteria occur in rows of similarly sized cells that form filaments called trichomes. Many trichomes are enveloped in a firm casing, but in Oscillatoria the casing is thin or may be absent altogether, which allows the filaments to glide quickly forward, backward, or even rotate. Some species of Oscillatoria can fix nitrogen but, unlike Trichodesmium, they may not have cells specialized for the purpose. Fragments of filaments, called hormogonia, which consist of dozens of cells, sometimes break off and glide away to establish new colonies. These bacteria may cause skin irritations in humans who come in contact with them in tropical waters.