62 LEECHES 



suggested that the critical oxygen tension below which the 

 oxygen uptake of an animal falls rapidly is the tension at 

 which the metabolism changes from a mainly aerobic to a mainly 

 anaerobic type. 



There is clear evidence from the work of Braconnier-Fayemendy 

 (1933) that Hirudo effects a transition to anaerobiosis, for in the 

 absence of oxygen there is a fourfold increase in the carbon content 

 of the urine. Moreover, there is no evidence of the repayment of 

 an oxygen debt after anaerobic metabolism (Hiestand and Singer, 

 1934) so the products of anaerobiosis must have been excreted. 

 In the other leeches whose oxygen uptake in relation to oxygen 

 tension has been studied it is probable that all are able to carry out 

 anaerobic metabolism, but those best adapted to life in low oxygen 

 concentrations are those which can maintain aerobic metabolism 

 to the lowest level of oxygen in the environment. 



When leeches find themselves in water of low oxygen content 

 they tend to move to the surface. In extreme conditions they pro- 

 trude the anterior half of the body from the surface of the water, 

 thus in effect carrying out aerial respiration. During the nineteenth 

 century medicinal leeches were kept in jars and if the leeches were 

 restless or rose to the surface this was taken as a forecast of bad 

 weather. The explanation of this phenomenon is not clear, 

 presumably they responded to falling barometric pressure. It has 

 recently been shown that earthworms and many other organisms 

 show a correlation between respiratory rate and barometric pres- 

 sure (Brown, 1957) so the leeches may have responded directly 

 to pressure changes, but a fall in atmospheric pressure will also 

 lead to a small fall in the concentration of dissolved oxygen in the 

 water, so it is also possible that the leeches responded to this, and 

 only indirectly to the pressure changes. 



