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The role of the cerebral organs of Paranemertes in salinity stress tolerance was investigated by measuring the effect of cerebral organ removal on volume regulation, and by observing the effects of hypo-osmotic media on the cytology of the cerebral organs. Removal of the cerebral organs decreases the capacity for volume regulation, however, a similar change is seen in sham-operated worms, indicating that the decreased capacity for regulation is due to the operation itself and not to interference with a physiological role of the cerebral organs. Cytological changes caused by in vivo exposure to dilute sea water are similar to those seen in worms fixed in hypo-osmotic fixative and therefore do not likely represent a coordinated response of the organs to salinity stress. Also, no exchange of material between the cerebral organs and the vascular system, as reported in some other species (Ling, 1970;Ferraris, 1979), was observed. We conclude that in Paranemertes, the cerebral organs are not involved in osmoregulation.t Present address: