104 THE PHYSIOLOGY OF EARTHWORMS 



of low internal oxygen tensions, but would not be of great sig- 

 nificance at low atmospheric tensions when there would be an 

 insufficient gradient to load the haemoglobin. In such a case it 

 would be expected that carbon monoxide w^ould have a relatively 

 greater inhibitory effect on respiration at high rather than low 

 oxygen tensions as shown by Johnson (1942). Considerable 

 amounts of oxygen are carried in physical solution at high oxygen 

 tensions, however, and the eifect of carbon monoxide may be 

 diminished by this fact. This has been demonstrated by Kriiger 

 and Becker (1940). 



Data for the aquatic oligochaete Tiihifex indicates that 50% 

 saturation is reached at 0-6 mm Hg oxygen thus making a steep 

 concentration gradient across the body wall. 



The function of haemoglobin as a carrier of oxygen is also con- 

 firmed for three tropical species, Protoscolex, Glossoscolex and 

 Pheretima hawaymia, the respiration of which is depressed by about 

 50% when the atmosphere contains 20% carbon monoxide 

 (Mendes and Valente, 1953). 



Chemical and Physical Properties of Oligochaete Haemoglobin 



The haemoglobin molecule of L. terrestris is a large one, with an 

 estimated molecular weight of 2,946,000, having a sedimentation 

 rate in the ultracentrifuge of 60-8 mm^x lO-^^^/sec/dyne at 20 °C. 

 On the basis of the porphyrin ring structure of haemoglobin the 

 molecule represented in Lumbricus blood contains 144 iron 

 atoms in some seventy-two units of 34,500 mol. wt. The iso- 

 electric point of the protein is pH 5-28 and upon hydrolysis this 

 breaks down to give at least six amino acids (see p. 5), Svedberg 

 (1933). 



Haemoglobin can be sedimented from blood plasma by ultra- 

 centrifugation at 76,000 g. The material is soluble in water and on 

 standing for one week at °C and pH 7 it turns brown with the 

 appearance of an absorption peak at 645 m/x, corresponding to the 

 formation of ferrihaemoglobin which disappears on adding 

 Na2S04. Two spectral bands are usually shown by solutions of 

 haemoglobin (Table 16). 



The reduction of the oxy-form of haemoglobin causes a slight 

 shift in the absorption spectrum as shown for Pheretima in Table 

 16. The time taken for the haemoglobin to unload half of its 



