Acclimatization of the freshwater field crab, Paratelphusa, to high temperature results in a decrease in the chloride, free amino acids and osmotic pressure of the blood.

Following similar acclimatization the freshwater mussel, Lamellidens marginalis unlike the crab, shows a considerable increase in the blood chloride as well as the free amino acids, while the total osmotic pressure increases relatively little.

These results are discussed and it is suggested that the ionic and osmotic gradient between the milieu intérieur and the protoplasm of the cells might be important in the metabolic compensation to temperature.

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