BLOOD AND ITS CIRCULATION 



43 



The story of haemoglobin in Daphnia is made more complicated 

 by the presence of an allied pigment in the gut. This is a haemo- 

 chromogen called daphniarubin. Haemochromogens of various 

 sorts can be made from haemoglobin by suitable chemical treat- 

 ment, and it is reasonable to assume that the pigment in the gut of 

 Daphnia is derived from its haemoglobin. But there is no simple 

 relationship between haemoglobin in the blood and haemo- 

 chromogen in the gut, so that we cannot be certain that Daphnia 

 makes such a conversion, except in one particular situation. We 

 have already mentioned that Daphnia passes haemoglobin into its 

 eggs; it has been noted that the concentration of haemoglobin 

 decreases during embryonic development, and that once the gut of 

 the embryo is formed haemochromogen begins to accumulate there 

 in increasing amounts. In the embryos it would seem that haemo- 

 globin is converted into haemochromogen. 



A summary of the changes involved in the synthesis and break- 

 down of haemoglobin in Daphnia is given in fig. 25. 



Only one other crustacean with haemoglobin in the blood has 

 been studied in detail comparable with the studies on Daphnia. 

 This is the brine shrimp, Artemia salina. In this species the con- 

 centration varies in much the same way as in Daphnia: there is more 

 haemoglobin when oxygen is scarce. The amount of salt in the 

 water where Artemia lives is often very high, and when this 

 happens the oxygen content of the water is low, so that a false 

 correlation appears between the salinity of the water and the haemo- 



FIC2S 



Fig. 25. Diagram showing the main routes of transference during 

 the synthesis and breakdown of haemoglobin in Daphnia. 



