Abstract
Several species of the genus Cryptomonas EHRENB. are characterized by the sigmoid (S-shaped) form of their cells under natural conditions. Such species were sampled and studied by the author for decades from the plankton of freshwater lakes, dam reservoirs, fishponds, and peat pools, from among littoral vegetation, as well as in small water bodies overgrown with vegetation. After application of optical microscopes on live or chemically treated specimens, they were measured and depicted. Four species known from literature are demonstrated to be easily determinable, namely Cryptomonas curvata, C. reflexa, C. marssonii, C. gracilis. Two new varieties are described. Cryptomonas borealis is shown as a transient species between straight and sigmoid cells. A special attention is given to the occurrence of pyrenoids and Maupas corpuscles within the cells. The new forma of C. cylindrica is pictured and described, with the antapex of the cells deformed differently from sigmoid species.