146 TRANSIT TON TO ANMJh'OBIOSIS 



the critical point along the scale of oxygen tensions, and 

 the seemingly qualitative difference between the two 

 groups is reduced to a merely quantitative one. 



It should be emphasized, furthermore, that the critical 

 point is by no means constant in a given species but is 

 influenced by external conditions {e.g.^ temperature) and 

 perhaps also by internal conditions. It is higher at high- 

 er temperatures, as has been repeatedly observed (Gaard- 

 er, 1918; von Buddenbrock and von Rohr, 1922; Linde- 

 man, 1935). In fact, one may expect that all the fac- 

 tors increasing or decreasing the rate of metabolism will 

 affect the position of the critical point (von Buddenbrock, 

 1939). 



The tension at which the oxygen consumption of some 

 organisms begins to decline varies considerably in the 

 experiments of various authors, especially in experi- 

 ments with worms: Tubifex (Dausend, 1931; Harnisch, 

 1935), the earthworm (Konopacki, 1907; Dolk and van 

 der Paauw, 1929; Thomas, 1935; Kriiger, 1938), the plan- 

 arians (Lund, 1921; Hyman, 1929; Fraps, 1930; Buch- 

 anan, 1931) and Nereis (Amberson, Mayerson and Scott, 

 1924; Hyman, 1932). An investigation on metazoa of 

 the factors mentioned by Kempner (1937, of, above) 

 might clear up some of these discrepancies. So far the 

 only observation which may account for such differences 

 is that of Harnisch (1935a) who found that the oxygen 

 consumption of Tubifex (and of other organisms as well) 

 showed a greater dependence on the tension when the 

 animals w^ere kept, prior to the determinations, in sur- 

 roundings poor in oxygen than when they were kept in 

 well-oxygenated water. However, this observation could 

 hardly explain all the other cases mentioned above. 



One other point should receive more attention in future 

 investigations. Some of the data that seem to indicate 

 an increase in oxygen consumption at tensions exceeding 

 that found in the atmosphere may not be quite trust- 



