Abstract
This study concerns the two species Trachelomonas volzii and T. dubia which were examined for similarities and dissimilarities of their loricae and monads organization. We specifically focused on the key features of both species that were originally used to separate one from the other: annular thickening at the base of collar and dimensions and ornamentation of loricae. Loricae of specimens were examined by light and scanning electron microscopy and the results were compared with the literature data describing these taxa and reported occurrences in the world. The species together with their varieties and forms do not appear to have strong characteristics distinguishing them, rather the variability observed fits natural phenotypic changes. Based on evidence from this study, we recommend combining these two taxa and propose T. volzii as the single taxon. We also examined a set of varieties of original T. volzii since the species contained several varieties that were almost identical in relation to lorica structure and occurrence. As a result of these observations, we propose the following varieties: Trachelomonas volzii var. volzii as a nominative variety, T. volzii var. australis, T. volzii var. sulcata, T. volzii var. inflata, T. volzii var. acidophila. Furthermore, we propose reclassifying some taxa and the new combinations such as: T. dubia var. ornata to T. volzii var. ornata and T. dubia var. colliundulata to T. volzii var. colliundulata. In our opinion, T. dubia fo. acuminata should be included with the species T. hexangulata due to its unique, hexagonal shaped lorica.