TRANSITION TO ANAEROBIOSIS 153 



excretion. A study of Nereis in this respect may explain 

 why Hyman (1932) observed no increased oxygen con- 

 sumption in these animals when they were transferred 

 to oxygen-saturated water after having first been kept 

 for one or more hours in oxygen-poor water. Not only 

 was there no increase, but the oxygen consumption re- 

 mained at an even lower level during the recovery pe- 

 riod than had been found during the initial period in 

 oxygen-rich water preceding the partial deprivation of 

 oxygen. 



Hall (1931) also found very little indication that an 

 oxygen debt accumulated in Urechis when the animals 

 were kept in water which sharply reduced their oxygen 

 intake. 



2. TBANSITION CAUSED BY RESPIRATORY POISONS OR BY THE SALINITY 

 OF THE MEDIL'M 



A. Respiratory poisons. The oxygen-carrying func- 

 tion of haemoglobin can be blocked by suitable concen- 

 trations of carbon monoxide without considerable inter- 

 ference with the ability of the tissues to perform aerobic 

 oxidations, that is, to utilize the oxygen that reaches 

 them. One finds then that the dependency of the oxy- 

 gen consumption on the tension begins at higher tensions 

 than normally. The reason is that, at reduced tensions, 

 only the respiratory pigment could insure a sufficient 

 oxygen supply; if the pigment action is blocked, the ani- 

 mals must get along with what oxygen reaches them by 

 diffusion. 



In the case of animals which, in normal conditions, 

 shift partially to fermentations in surroundings very 

 poor in oxygen, the transition will take place earlier, i.e., 

 at higher oxygen tensions, when the specimens are 

 poisoned. Experiments along that line have been per- 

 formed with the earthworm (Jordan and Schwarz, 1920; 

 Dolk and van der Paauw, 1929; Thomas, 1935; Kriiger, 



