Tetrahymena setifera n.sp., a Member of the Genus Tetrahymena with a Caudal Cilium*

SUMMARY. A new species of Tetrahymena is described, the ninth to be allocated to this genus. Most significant morphologically and taxonomically is its possession of a single, slender caudal cilium, twice the length of the surrounding somatic ciliature. The presence of this inconspicuous organelle, quite novel to the genus, is also detectable in the infraciliature where an argentophilic “polar basal granule‐complex” appears exactly at the posterior pole of the body. The diagnosis of the genus Tetrahymena is expanded to embrace species with such a caudal appendage.