1. The oxygen consumption of Sabella decreases soon after the concentration of dissolved oxygen in the sea water falls below the value corresponding to air saturation both at 10 and 17 degrees C. Above air saturation at 17 degrees C there is no significant rise in oxygen consumption. 2. The oxygen consumption of Sabella whose chlorocruorin has been converted to carboxychlorocruorin is lower than that of normal worms. This is the case at air saturation of the water and at oxygen concentrations below air saturation both at 10 and 17 degrees C. 3. The fall in oxygen consumption of the animals after treatment with carbon monoxide is not due to an action of the latter on cell enzymes. 4. It follows that chlorocruorin functions as an oxygen carrier in Sabella at all temperatures and oxygen pressures to which the animals are subjected in nature.