The Osmotic Concentration of the Blood of Callorhynchus millii and Epicercctodus (Neoceratodus) forsteri, and the Significance of the Physico‐chemical Condition of the Blood in regard to the Systematic Position of the Holocephali and the Dipnoi.

Summary. The osmotic pressure and the salinity of the blood of Callorhynchus antarcticus and of Epiceratodus forsteri have been investigated with a view to determining the relations between internal fluids and the aquatic environment in the Holocephali, and the Dipnoi. Live fish were examined, and the specimens were quite normal (the Callorhynchus were examined at Hobart, Tasmania, and the Neoceratodus at Brisbane, Queensland). The blood of the Callorhynchus had a freezing‐point almost the same as the sea water in which the specimens were living. The blood contained over 2·6 per cent. urea and chlorine representing 1·33 per cent. NaCl. These figures indicate that Callorhynchus presents the extraordinary conditions found in only one group of the animal kingdom—the Elasmobranchii. The blood of Epiceratodus has a relatively high freezing‐point, —0·42°C., compared with —0·5 to 0·6 for freshwater Teleostei, and —·56 to —0·6 for mammalia. There is no resemblance whatever to Elasmobranch conditions. The Dipnoi fall into the same category as the higher vertebrates. The freezing‐point of the blood is curiously like that of the amphibia.