Tachysoma pellionellum Stokes, 1887, a freshwater ciliate isolated from Stone Mountain State Park, North Carolina, was studied in vivo and after staining with protargol. The population was characterized mainly by having the typical 18 frontal-ventral-transverse cirri; posterior ends of left and right marginal rows not confluent; five dorsal kineties and one dorsomarginal kinety; two macronuclear nodules near left cell margin with one or two micronuclei between them; contractile vacuole located at mid-body near left margin. Morphogenesis is characterized as follows: (1) in the proter, the parental adoral zone of membranelles is retained completely; (2) 18 frontal-ventral-transverse cirri are derived from the anlage of the undulating membrane and the five streaks of the frontal-ventral-transverse anlagen; (3) marginal rows develop intrakinetally; (4) anlagen of dorsal kineties 1, 2 and 4 develop in the parental structure and anlagen of dorsal kineties 2 and 4 fragment in the posterior region forming anlagen of dorsal kineties 3 and 5; (5) only one dorsomarginal kinety formed; (6) the two macronuclear nodules fuse into a single mass, which then divides. Phylogenetic analyses based on sequences of the gene coding for SSU RNA revealed a close relationship between T. pellionellum and the Oxytricha clade, both of which grouped with Kleinstyla dorsicirrata and Heterourosomoida lanceolata.
