Computing the direction and amplitude of orienting and avoidance turns is fundamental to prey pursuit and risk avoidance in motile foragers. We examined computation of turns in the predatory sea-slug Pleurobranchaea californica, observing orienting and aversive turn responses to chemotactile stimuli applied to the chemosensory oral veil. We made seven observations: (1) the relation of turn angle/stimulus site on the oral veil was linear; (2) turn amplitudes increased with stimulus strength; (3) turn responses markedly overshot the target stimulus; (4) responses to two simultaneous stimuli at different loci were averaged to an intermediate angle; (5) stimuli could induce sequential turns in which the angles of the first and third turns were similar, a form of working memory; (6) turn direction was affected by appetitive state, so that animals with higher feeding thresholds tended to avoid appetitive stimuli; and (7) avoidance turns induced by mildly noxious stimuli were computed similarly to orienting, while differing in direction. These observations appear to outline a framework of behavior that could be employed for efficient tracking of odor trails, and which is regulated by decision mechanisms that integrate sensation, internal state and experience.