OXYGEN CONCENTRATION AS A LIMITING FACTOR 



IN THE RESPIRATORY METABOLISM OF 



PLANARIA AGILIS. 



E. J. LUND, 



DEPARTMENT OF ANIMAL BIOLOGY, 

 UNIVERSITY OF MINNESOTA. 



Living organisms have been conveniently grouped into ob- 

 ligate aerobes, facultative aerobes, facultative anaerobes and 

 obligate anaerobes. The criterion for this classification is 

 fundamentally the relation of organisms to the concentration of 

 free oxygen. Free oxygen by its concentration effects on the 

 metabolism acts as a limiting factor in determining animal and 

 plant distribution. While the role of oxygen concentration in 

 the life of lower plants and animal parasites has been known in a 

 general way for a long time, yet the importance of differences in 

 oxygen concentration as a factor determining the differences in 

 distribution of many invertebrate animals, which we would 

 ordinarily consider aerobic, has not received the emphasis which it 

 deserves. The grouping of organisms into the above four groups 

 is merely arbitrary, for from the small amount of data available it 

 is clear that some organisms stand in an intermediate position 

 between the above groups. 



Since the rate of respiration is correlated to such conditions as 

 state of nutrition, size and age, regeneration, cell division and 

 irritability, it is logical to suppose that by comparing the effects 

 of different oxygen concentrations in the different states of 

 nutrition, age, regeneration and so on, valuable information 

 might be obtained which would throw further light on the 

 relation of respiration to this group of correlated processes . 



In the present paper three questions w r ill be considered in so 

 far as they apply to Planaria agilis: (i) What is the quantitative 

 relation between oxygen concentration and the rate of oxygen 

 consumption? (2) What is the effect of the state of nutrition 

 upon the relation of oxygen concentration to rate of oxygen 



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