Chemo‐orientation

Summary 1. A new classification system for chemo‐orientation mechanisms is suggested, based on delineating (a) internal and external orientation information available to an organism, (b) mode of information processing, (c) type of motor output pattern, and (d) guidance systems that modulate or direct motor output. 2. Cross‐phyletic comparison of chemo‐orientation indicates that morphology, to a greater extent than phylogeny, determines the kind of chemo‐orientation mechanisms available to an organism. Analogies are drawn between single‐celled organisms with surface chemical receptor sites and higher organisms with elaborate chemosensory organs and nervous systems.