| Summary: | 1. The temperature dependence of diffusional permeability to water was studied in freshwater-adapted and seawater-adapted eels. The pattern of temperature acclimation is discussed. 2. The ratio of water permeability to sodium permeability is approximately 3 at 25°C, but falls to 1 at 5°C. This drop is mainly due to a diminution of the water permeability, the temperature coefficients being much higher for water than for sodium. 3. The relatively independent variations of water and sodium permeabilities in the seawater-adapted fish probably indicate a certain dissociation between water movements and salt movements. 4. In the freshwater-adapted eel the osmotic permeability is considerably higher than the diffusional permeability, which supports the previously advanced hypothesis concerning the presence of water-filled channels in the branchial epithelium. 5. In the seawater-adapted eel the osmotic permeability is lower than the diffusional permeability, this difference being greater the lower the temperature. This surprising result must signify either that the osmotic pressure difference between blood and sea water does not represent the true osmotic gradient across the membrane, or that a reabsorption of water linked with a movement of solutes occurs in a specialized region of the gill.
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