208 E. J. LUND. 



oxygen concentration may act as a limiting factor in the rate of 

 oxygen consumption and shows also what are the quantitative 

 relations. 



This result is similar to those reported by Henze (10) for 

 Anemonia sulcata, by Thunberg ('05) for Limax and Lumbricus, 

 and in the leech as found in this laboratory by Miss Wolf. In 

 a previous paper by Lund ('18) it was shown that the rate of oxy- 

 gen consumption by Paramecium caudatum is practically entirely 

 independent of the concentration of oxygen. This relation would 

 be represented in Fig. 2 by a straight line drawn from some 

 appropriate point on the y-axis parallel to the x-axis. Evidently 

 the oxidation reactions in Paramecium and Planaria are quite 

 different in certain respects. This is further shown by the fact 

 that the inhibitory action of KNC on the rate of oxygen con- 

 sumption in Planaria is very marked, as shown by Allen ('19, !.) 

 while a similar inhibitory action on the oxygen consumption in 

 Paramecium caudatum could not be found, Lund ('18, II.). 4 



It may well be that we shall be able to find in the results of 

 studies on the relation of oxygen concentration to the rate of 

 oxygen consumption, a key to the solution of the question as to 

 why it is that the action of KNC upon Planaria agilis and Para- 

 mecium caudatum is different in character. Work is under way 

 on this question. 



It must be kept in mind that since we do not as yet have any 

 accurate data on the magnitude of the diffusion coefficients of 

 oxygen through the protoplasm of cells and different tissues, it 

 is really impossible to say whether in such a case as Planaria 

 for example, the decrease in the rate of oxygen consumption at 

 lower oxygen concentrations is due to slow rate of diffusion of 

 the oxygen into the interior of the animal, which rate is itself a 

 function of the difference of oxygen pressure between the inside 

 and outside of the animal, or whether the difference is actually 

 due to an inherent difference in reaction velocity as expressed 



4 Various kinds of criticisms of these experiments have been made by Child and 

 Hyman. A careful consideration of the objections offered has however entirely 

 failed to disturb my confidence in the results as given in the experiments, and the 

 correctness of the conclusions drawn from them. A more complete consideration 

 of the criticisms with further experimental data is given elsewhere. Lund ('21.) 



