SURVEY OF INVERTEBRATES 63 



rarily be confined to the real anaerobic zone. It is quite 

 likely, however, that they will occasionally be subject- 

 ed to complete anaerobiosis. On the whole, the sapro- 

 pelic worm fauna should show pronounced adaptation for 

 life at quite low oxygen tensions and future investiga- 

 tion may even reveal that some species can live and re- 

 produce in the complete absence of oxygen. An experi- 

 mental approach to this problem is much desired. 



Another anaerobic or nearly anaerobic habitat in which 

 worms are also found is the bottom of polluted lakes or 

 streams, or that of lakes in which the circulation has 

 ceased. Tubificidae are especially characteristic of such 

 environments. They are frequently present in greatest 

 numbers just in the most heavily polluted areas (Hent- 

 schel, 1917; Richardson, 1925) where only minute amounts 

 of oxygen are present. In uncontaminated and non-circu- 

 lating bodies of water where the oxygen disappears com- 

 pletely during summer- or winter stagnation they must 

 undergo total anaerobiosis. 



The Tubificidae are nevertheless not animals that can 

 live indefinitely in the complete absence of oxygen. They, 

 like many other organisms in this type of habitat, endure 

 anaerobic conditions, but do not select them (Eggleton, 

 1931). Alsterberg (1922) described a very interesting 

 respiratory mechanism in these worms, in particular, in 

 Tubifex and Limnodrilus. While they may actually be 

 buried in oxygen-free mud, their tails usually project 

 into the water above it. If there is much oxygen in the 

 water, the tail is kept motionless, but as soon as the 

 oxygen content of the water in contact with the mud be- 

 comes insufficient, the worms begin to wag their tails and 

 to thrust them the farther out of the mud the lower the 

 oxygen content of the water becomes. In this manner oxy- 

 gen is drawn into the "anaerobic" zone and becomes 

 available to the worms. At very low tensions feeding 

 ceases; Limnodrilus then becomes quiescent, while Tubi- 

 fex tries to escape asphyxia by wandering away. 



