Oxytricha acidotolerans sp. nov. after protargol impregnation. a, b: Infraciliature of ventral and dorsal side and nuclear apparatus of early divider. As is usual, anlagen III (short arrow) and IV (long arrow) originate from disorganised parental cirri III/2 and IV/3, respectively. In the area where the nuclear apparatus is located, the oral primordium is not clearly recognisable. c, d: Infraciliature of ventral and dorsal side and nuclear apparatus of middle reorganiser. Arrow in (c) marks new left frontal cirrus originating, as is usual, from anlage I. Asterisks denote marginal row anlagen. Dotted line circles frontoterminal cirri (VI/3, VI/4) which are, as is usual, not involved in primordia formation. Arrow in (d) marks new caudal cirrus on dorsal kinety 1. Obviously this specimen has an additional old and new (double arrow) dorsal kinety 4. MA, macronuclear nodule; MI, micronucleus; OP, oral primordium; RMR, anteriormost cirrus of old right marginal row; VI, frontal-ventral-transverse cirri anlage VI; 1–6, dorsal kineties. Scale bars: 30 μm.