64 SUR\ HY OF /\\ HirrE/i/y\\T/JS 



Field observations indicate that in nature the Tubifi- 

 cidae survive relatively long periods under truly anaero- 

 bic conditions. They are still found, although in re- 

 duced numbers, in the bottom material of lakes of which 

 the water has been found free of oxygen for several 

 weeks (Juday, 1908, 1922; Lindemau, 1942). In this con- 

 nection the quantitative figures presented by Lindeman 

 (1942) are of special interest. He found in mud sam- 

 ples of Cedar Bog Lake, Minnesota, the following num- 

 bers of Tubifex per Birge-Ekman sample of 225 cm' bot- 

 tom mud : 



January 31, 1940 (7 days before onset of anaerobiosis) : 6.0 ±:^ 1.8 

 March 3, 1940 (24 days after onset of anaerobiosis): 5.2 zt 0.5 



March 30, 1940 (51 days after onset of anaerobiosis) : 1.0 rt 0.7 



Rotatoria seem to occur occasionally in relatively large 

 numbers in the sand of beaches of fresh water lakes where 

 the interstitial water has a very low oxygen content or 

 is even completely free of oxygen. Pennak (1940) found 

 in such habitats from 79 to 130 rotatoria per 80 cc. of 

 sand. The predominant species were apparently Lecane 

 inquieta, L. paraclosterocerca and L. sciitata. Moore 

 (1939) encountered Rotaria rotatoria and R. tridens in 

 the oxygen-free bottom ooze of Douglas Lake, Michigan, 

 where also the nematode Trilobus bastian and a species of 

 Chaetonotus occurred. Gastrotrichs, in general, seem to 

 live rather frequently in similar localities (Juday, 1908; 

 Imel, 1915). 



Sewage plants might well be inhabited by worms able 

 to live in the absence of oxygen. But no conclusive evi- 

 dence from the literature that such is the case has come 

 to the writer's attention. Reynoldson (1939), on the other 

 hand, has pointed out that Enchytraeid worms which do 

 occur in sewage plants must be regarded as living aero- 

 bically. 



(b) Marine forms. It has been shown pre- 

 viously that the bottom material of the sea is frequently 

 free of oxygen. Let us consider the possibilities which 



