74 



COMPARATIVE PHYSIOLOGY 



man. The former has a much lower affinity for carbon 

 monoxide than the latter. At the point of 50 per cent, satura- 

 tion under exposure to a 

 mixture of oxygen and 

 carbon monoxide, the par- 

 tial pressures of CO and 

 O2 were not 250 : i as in 

 the case of man, or 140 : i 

 as in the mouse, but more 

 nearly 40:1. Again, as 

 regards the blood spectra, 

 it was found that the a- 

 oxy-hsemoglobin band is 

 18 Angstrom units nearer 

 the violet end, and the a- 

 carboxy-hsemoglobin band 

 is 1 1 Angstrom units nearer 

 the violet end than are 

 those of human blood. 



100 



1^80 



S 60 

 g 50 



-g 30 

 0^20 



10 



Pressure of O2 , rams. 



~A=pU. 7*3 ; B=^^H. 6*9 ; Temp. 20° C. 

 C and D human blood, C=20°C. ; D = 

 37°C.;^H.=7-45. 



Fig. 17. — Dissociation curves of areni- 

 colan blood (Barcroft and Barcroft). 



Further, Vies has shown that the methaemoglobin of Areni- 

 cola has not the typical bands in the spectrum of mammalian 

 haemoglobin. There is a quantitative relation (Barcroft) 

 between the affinities of the different haemoglobins for carbon 

 monoxide and oxygen, on the one hand, and the position of 

 the bands in their absorption spectra on the other. The 

 logarithm of the reciprocal of the pressure at which the pigment 

 is half saturated with each is a linear function of the situation 

 of the a-band. The further the band is situated towards the 

 violet end, the greater in each case is the affinity for the 

 gas. From measurement of the blood- volume and oxygen- 

 consumption of Arenicola, it appears that Arenicola blood 

 can store just about enough oxygen to last when, sealed up 

 in its burrow at low tide, it has temporarily no access to 

 water. 



We may now turn to the consideration of the respiratory 

 pigment of molluscs and Crustacea. Haemocyanin is a term 

 given to a family of substances which in the presence of 

 oxygen display a bluish hue, are like the haemoglobins of pro- 



