The morphology and morphogenesis of the hypotrich ciliate Urosoma macrostyla (Wrześniowski, 1866) Berger, 1999, collected from a puddle in Harbin, China, were investigated using live observation and protargol impregnation. Based on previous and present studies, an improved diagnosis of U. macrostyla is supplied. It differs from its congeners mainly by the body shape, no cortical granules and number of macronuclear nodules. The ontogenesis of U. macrostyla is typical for species with such a somatic ciliary pattern: the oral primordium develops hypoapokinetally and FVT-anlagen develop in 5-streaks and primary mode. However, a unique characteristic in morphogenetic process is reported: anlagen for both the left and right marginal cirri occur de novo to the right of the parental structure which has never been seen in other oxytrichids. This characteristic was considered an apomorphy (Berger 1999). This indicates that U. macrostyla possibly has a high phylogenetic position within the genus Urosoma, or perhaps it represents a distinct subgenus.
