FUNCTIONS OF BLOOD 



105 



days at lower tensions. In their natural habitats the Chiro- 

 nomid larvae can stand much lower oxygen tensions and per- 

 haps even complete oxygen lack (Juday, 1908), probably by 

 changing over partially or 

 completely to anaerobic me- 

 tabolism (Harnisch, 1936). 



Willmer (1934) describes 

 the very peculiar situation 

 of fishes in tropical swamps 

 in British Guiana. The 

 oxygen is so low (and often 

 practically absent) that the 

 fishes have had to develop 

 accessory organs for breath- 

 ing air. At the same time 

 they are exposed to water 

 with a C0 2 tension which 

 is sometimes very high. In 



these circumstances their dissociation curves are not very 

 steep, but the C0 2 effect is very much reduced (Fig. 60). 



In one fish, the yarrow (Erythrinus) , Willmer studied the 

 complex relations between the gas content of the water and 



20 30 40 50 60 70 80 90 100 

 Oxygen pressure in mm. Hg 



Fig. 60. Oxygen dissociation curves 

 for blood of electric eel at C0 2 tensions 

 from to 25 mm. (Willmer.) 



CO, too high|Aeidaemia(?)) 



Gill operculum actively 



closed 



. u . :• kept vntu" 



C0 2 content too lo 



I 2 3 4 5cc. per litre 



27 54 , 81 108 I35m-.H« 



Oxygen pressures and oxygen content of water at 26 C. 



Fig. 61. Diagram to illustrate the respiratory behaviour of the yarrow in 

 water with different concentrations of 2 and C0 2 and its possible explanation. 

 See text. (Willmer.) 



