Co-luciferase is prepared (1) by heating a luciferase solution to 65° (1917 _a_) or (2) by extracting with water portions of the siphon of _Pholas_ which have previously been macerated and well extracted with alcohol (1918 _a_). Long-continued treatment with alcohol apparently destroys the luciferase without affecting the co-luciferase. On mixing a solution of preluciferin with one of co-luciferase and allowing them to stand for 8-10 hours, luciferase is formed and can be recognized by the fact that it will give light with a crystal of KMnO_{4}. Preluciferine does not do this. Recently Dubois (1918 _a_) states that preluciferine is nothing but taurine and that taurine occurs in large quantities in _Pholas_ and is transformed into luciferine by the action of co-luciferase. Not only taurine, but also Byla's peptone, egg lecithin, and esculin can be converted into luciferine by co-luciferase, and since esculin, a glucoside, is so transformed, Dubois believes this proves that co-luciferase belongs to the hydrolases. Indeed, it proves too much. Luciferin must have an extraordinary chemical structure if it can be formed by hydrolysis of such diverse compounds as peptone, lecithin, esculin and taurine. A glance at the structural formula of esculin and taurine is sufficient to emphasize the diverse nature of these two substances. [Illustration: Taurine] [Illustration: Esculin] I believe that in these experiments Dubois has been working with an oxidation product of luciferin, what I have called _oxyluciferin_, rather than a pro-substance. The mode of preparation of _Pholas_ preluciferin and _Pholas_ co-luciferase is such as could be used in the preparation of _Cypridina_ oxyluciferin, and it seems more logical to look for the presence of _Pholas_ oxyluciferin in one or both of Dubois' extracts rather than believe that luciferin can be formed from both taurine and esculin. When the co-luciferase solution stands with the preluciferin solution we would in reality have not the formation of luciferin from preluciferin, but the formation of luciferin from oxyluciferin, by some reducing agent in the mixture. Indeed, in a very recent paper Dubois (1919 _c_) takes the view that his co-luciferase is a reducing enzyme which forms luciferin by reduction (presumably from oxidized luciferin) and no mention is made of preluciferin.