Some integrated patterns of behavior have been studied in colonies of Veretillum cynomorium, whose various parts often act as if they were those of an individual consisting of the whole colony. The entire colony is involved in slow rhythmical movements, in which phases of contraction alternate with expansion. This behavior corresponds to a permanent reciprocity (or duality) between the zooid-bearing rachis and the peduncle; overall behavior is determined by the number of “functionally active units” which either contains at any given time. This number is influenced by the excitatory or restraining effects of external factors such as light intensity or the entry of water. In this way, when contraction proceeds, activity of the rachis predominates, whereas during swelling of the colony, activity of the peduncle prevails. Observations on intact colonies and on specimens without peduncles, experiments using light and electrical stimuli, and the study of peristalsis, all suggest that the mutually reciprocal nature of activity in the rachis and peduncle may be attributed to the differential influence of external factors on the functional state of centers of spontaneous activity or of conduction pathways in the colony. Zooids present on the rachis are responsible both for the reciprocal activity of the colony and for its morphological polarity.