Stentor and Blepharisma possess photoreceptor systems that render the cells capable of avoiding light. The observed photobehaviour is largely the result of a step-up photophobic response displayed by both ciliates. The photophobic response caused by a sudden increase in light intensity starts with a cessation of ciliary beating, then a period of ciliary reversal followed by renewed forward movement. Ciliary reversal correlates with the generation of an action potential, which is elicited by a photoreceptor potential as a result of light absorption by cellular pigments.