RESPIRATION 93 



oxygen but the rear end of the body projects into the water flowing 

 above them. Fox and Taylor (1955) found that adult Tubifex 

 survive equally well in water containing 21% oxygen i.e. air 

 saturated, and in water with only 4% oxygen. Conditions such as 

 these may be expected to occur in situations suitable as habitats for 

 these worms. In the artificial conditions of complete saturation of 

 the water with 100% oxygen the animals die in a few days. No 

 details are given of the rates of respiration in these cases. 



The survival of adult Ttihifex then is not dependent upon the 

 provision of a high partial pressure of oxygen. The situation with 

 regard to newly hatched Tubifex, however, is very diflferent. The 

 hatching of cocoons occurs equally well in water containing 21% 

 oxygen, but after one month more worms were living in 4% 

 oxygenated water than in air saturated water. Moreover the indivi- 

 duals in 4% oxygen were larger, having a mean volume of 5-68 

 mm^, compared with 1-79 mm^ of those in air saturated water 

 (Fox and Taylor, 1955). 



The influence of complete oxygen saturation of water has also 

 been studied by Walker (1959) using pressures above atmospheric. 

 Eight hours treatment with 100% O2 at four atmospheres pressure 

 has no noticeable eff"ect on Tiihifex but more prolonged exposure 

 causes death, in much the same way as reported by Fox and 

 Taylor (1955). Interruption of the treatment prolongs the survival 

 time, suggesting that some recovery from oxygen damage is 

 possible. The provision of a high osmotic pressure externally in 

 the form of a salt solution promotes survival, as it also does in 

 cases of heat stress, carbon monoxide and hydrogen peroxide 

 poisoning. But the application of salt solutions to a fresh-water 

 dweller must itself have deleterious effects and provoke the 

 occurrence of homeostatic mechanisms to maintain internal 

 equilibria. 



Changes in the proportion of the body of adult Tubifex is 

 exhibited in response to oxygen variation. The river and lake- 

 bottom mud in which the oligochaetes live is also the home of many 

 bacteria and the activity of the latter may at times reduce oxygen 

 tensions virtually to zero. Such oxygen depletion is followed by 

 extension of the body of T. tubifex to a length ten to twelve times 

 normal, thus projecting the rear end away from the substratum into 

 water more likely to contain oxygen, and also presenting a greater 



