In a previous communication it has been shown that the haemocyanins of Crustacea and of Helix are fundamentally different with respect to the action of neutral salts, and the pH corresponding to minimum affinity for oxygen (Hogben and Pinhey, 1926). The object of this communication is to show that the haemocyanin of Limulus is different from that of either Helix or the species of Crustacea hitherto investigated. The method employed for studying the dissociation of oxyhaemo-cyanin in this research was a colorimetric procedure. Opportunity may here be taken of describing the more elaborate colorimetric method used for this purpose, though the principle is essentially similar to that indicated as a method for class work by students in a communication by Pantin and Hogben (1925). The general similarity in behaviour of haemocyanins and haemoglobins, the extreme simplicity of the method (even in the form described below) and the...

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