RESPIRATION IN WATER 29 



any effective water uptake per anum in the Polychata with the 

 possible exception of Owenia. In any case the volume of 

 water "respired" seems to be so small that a primary respira- 

 tory function is unlikely. In the large worm Aphrodite aculeata 

 Stephenson described a swallowing of water for respiratory 

 purposes which is again expelled through the anus, but ac- 

 cording to Lindroth (personal communication) this is a 

 mistake and the gills on the parapodia are the only respira- 

 tory organs. 



Alsterberg in 1922 described carefully the respiratory 

 mechanisms of the small threadlike fresh-water worms 

 Tubijicida, building tubes in very soft organic mud which is 

 practically CV-free and absorbs O2 at a rapid rate by the activ- 

 ity of micro-organisms. The behaviour of the worms depends 

 upon the oxygen content of the water just above the mud. 

 The gradient is usually very steep. Oxygen may be present 

 just above the mud in which case the animals behave as shown 

 in Fig. 8a. With increasing distance to water containing 



mmmm 



z_ 



Fig. 8. Diagram showing the reactions of Limnodrilus to diminishing 

 concentrations of oxygen. See text. (Alsterberg.) 



dissolved oxygen, the animals stretch out their tails farther 

 and farther and make rhythmic oscillatory movements by 

 which water containing oxygen can be brought down from a 

 vertical distance of a few cm. When this does not suffice and 

 the 2 concentration in the immediate vicinity of the animal 

 {Limnodrilus) sinks below 0.08 ml/1 (1.5 mm 2 tension), 

 reversible asphyxia is produced, and the worms become im- 

 mobile (Fig. 7f). Certain forms (Tubijex) will migrate from 

 the habitat before asphyxiation occurs. 



Alsterberg observed antiperistaltic movements of the hind 

 gut and believes that intestinal respiration plays a major part 



