BLOOD AND ITS CIRCULATION 41 



variations in the amount of oxygen in the water in which Daphnia 

 lives. When there is little oxygen the blood is bright red; when 

 oxygen is abundant the blood is pale. Individual animals can be 

 changed from pale to red, and back to pale by appropriate altera- 

 tions of the oxygen content of the water. The change from pale to 

 red take about ten days, and the reverse change proceeds at the 

 same rate. These changes are similar to those found in mountaineers 

 when they become acclimatised to rarified air. The concentration 

 of haemoglobin in their blood increases, and then decreases when 

 thev re-enter the denser air of the lowlands. The changes in Daphnia 

 are the more spectacular, for the concentration of haemoglobin 

 can increase twelvefold, whereas in man the concentration only 

 increases by about one-fifth. 



The function of haemoglobin in Daphnia has been demonstrated 

 in a variety of ways. If red and pale specimens are put in stoppered 

 bottles with water containing very little oxygen it is found that 

 the red specimens survive much longer than the pale specimens. 

 This indicates that the haemoglobin in the red specimens enables 

 them to utilise oxygen which is not available to the pale specimens. 

 This has been confirmed with another daphnid, Simocephalus 

 vetulus; red specimens continue respiring at a greater rate than 

 pale specimens when both are kept in low concentrations of oxygen. 



The haemoglobin in red specimens can be rendered functionless 

 by treating the animals with carbon monoxide. This gas combines 

 with haemoglobin and prevents it from taking up oxygen. Daphnia 

 which have been so treated are similar to pale specimens; they 

 survive a shorter time in poorly aerated water than do red, 

 untreated specimens. The treated individuals also move more 

 slowlv and collect less food than the untreated individuals, so 

 that it is clear that the haemoglobin in red specimens enables them 

 to gather more food than they could without the pigment. 



Although oxygen is the dominant factor controlling the con- 

 centration of haemoglobin in Daphnia, other factors do play a part. 

 In order to make haemoglobin Daphnia must be adequately fed, 

 and there must be a certain amount of iron in the water, for iron 

 is an essential part of the haemoglobin molecule. Temperature also 

 plays a part. When the temperature increases the chemical processes 

 of the body speed up — the metabolic rate is increased, and more 

 oxygen is required to satisfy the animals' needs. If Daphnia is in 

 poorly aerated water the increased demand for oxygen increases the 

 relative deficiency of oxygen, so that the stimulus to synthesise 



