The body water content of Theromyzon tessulatum changes considerably during the life cycle. Water content is low and undergoes little change during the first stages, but it strongly and regularly increases during the last stage prior to reproduction. It is controlled by a water balance factor (FRBH) released by the brain. With selective removals of nervous follicles, the site of production of this hormone has been localized in the posterior paramedial follicles (follicles 4).