STEP-UP AND STEP-DOWN PHOTORESPONSES IN BLEPHARISMA

The cells of Blepharisma responded to a step-up in light intensity (step-up response) by ciliary reversal, while the cells showed a temporal repression of ciliary reversal which was accompanied by swimming acceleration when light intensity was suddenly decreased (step-down response). The step-up response occurred only in anterior fragments obtained by bisection of the cells, while the step-down response occurred in both of fragments. The topographical difference in photosensitivity indicates that the photoreceptor systems responsible for the step-up photophobic response might be different from those for the step-down photoresponse (repression of ciliary reversal accompanied by acceleration of swimming velocity).