Photomicrographs of live A, B, E, F bouin’s fixed C and protargol-impregnated D Urocentrum turbo from PDC A–C and Sentino River population D and Indian populations E, F. A a live specimens of Urocentrum turbo B specimen about to burst after forming atypical structure as consequence of light exposure C specimen fixed with bouin’s fixative to avoid bursting (arrowheads mark the macronucleus) and record details D a non-cave dwelling population collected outside the cave (Sentino River) of the Frasassi cave, arrowheads marks the macronucleus. Note that the Sentino River specimens were found to be resistant to light exposure though deprived of trichocysts. E, F the Indian populations were found to be resistant to light exposure and possessed the trichocysts (arrows), double arrowheads mark the caudal cirri. Scale bars: 25 µm.