1. By means of the colorimetric method the effect of temperature, hydrogen ion concentration, and salinity upon the dissociation of the haæmocyanin of four species of decapod Crustacea (Maia, Cancer, Palinurus, and Homarus) has been studied.

2. Rise in temperature depresses the dissociation curve continuously between o° and 50° C. Reasons are given in favour of the conclusion that this behaviour is consonant with the applicability of the mass action law. On this understanding the heat of reaction between hæmocyanin and oxygen dissolved in the water phase in the case of Maia is of the order 9500 calories per n gram molecules of oxygen, n being defined as the least number of molecules of oxygen which can enter into the reaction.

3. In the case of all four crustaceans referred to above, the affinity for oxygen diminishes up to a point as the hydrogen ion concentration is increased : on further increasing the hydrogen ion concentration beyond a critical value for which the affinity of the serum for oxygen is minimal, the amount of oxygen taken up at low tensions increases and may surpass the values obtained for serum at normalpH. The similarity of this result with the observations of Rona and Ÿllpo on hæmoglobin is discussed.

4. On concentrating serum with neutral chlorides of the alakali and alkaline earth metals the dissociation curve is made steeper.

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