Ragworms,Nereis diversicolor,kept under constant conditions were spontaneously active and performed a number of different responses. The initial reaction to stimulation from a variety of chemical, mechanical, light and electrical stimuli was usually posterior movement in the tube, and, on at least some occasions, this was probably a withdrawal reflex. Apart from this, there is evidence that stimulation affected the worms’ behaviour by accelerating, and to some extent modifying, the occurrence of patterns of behaviour which occur spontaneously in unstimulated worms. The performance of searching and reversal responses was certainly modified by stimulation, and data is presented which supports the hypothesis that their occurrence is regulated by mechanisms which balance the signals of the results of action (i.e. reinforcements) received by the animal. Food presents a complex stimulus situation and, in reacting to it, worms came successively into contact with a series of stimuli. These elicit different (reflex) responses and this, together with their effects on the occurrence of searching and reversal responses, resulted in them performing a sequence of responses.