Cephalic mechanism for social control of fissioning in planarians: III. Central nervous system centers of facilitation and inhibition.

Previous studies have indicated that asexual reproduction (fissioning) in the planarian Dugesia dorotocephala is socially controlled through a cephalic mechanism: Isolation releases fissioning; grouping inhibits it; decapitation, at the level of the auricles, releases it even in grouped subjects. The brain is not necessary for programming the actual events of fissioning; these are orchestrated by the segmental plexus fissioning (SPF) system. Various surgical cuts were made to ablate selected portions of the central nervous system of isolated and grouped planarians in order to ascertain the inhibitory or facilitatory effects of these in the physiological mediation of such control on the SPF system. These results were synthesized into a model of this control system; the anterior lobes and optic regions of the brain inhibit the SPF system, and the anterior and caudal segmental plexuses facilitate it. These influences are partially tonic and partially contingent upon social stimulation.