68 SUIU'EY OF IWERrEBRATES 



that a parallelism exists between this resistance and the 

 occurrence of anaerobic conditions in the normal sur- 

 roundings of the worms. Thus we see a remarkable en- 

 durance exhibited by Tubifex and its allies, by mud- 

 dwelling nematodes and mesosaprobic rotifers like Colu- 

 rella, Rotifer or Philodina; while others, such as the oli- 

 gosaprobic rotatorians Synchaeta, Schizocerha, etc., die 

 rapidly in the absence of molecular oxygen. 



The minor differences in survival times of diiferent 

 worms do not appear to be really significant. Very 

 likely the figures given in Table 10 do not, in many cases 

 at least, represent the true resistance in anaerobic con- 

 ditions. In some of the anaerobic cultures, notably those 

 of Hecht (1932) and Nikitinsky and Mudrezowa-Wyss 

 (1930) rather considerable amounts of hydrogen sulfide 

 developed ; since this gas is toxic for many organisms, its 

 development may well mask the real anaerobic functions. 

 It seems obvious that small differences in anaerobic sur- 

 vival cannot be used with advantage for ecological specu- 

 lations. Rode's (1925) attempt to do so with planarians 

 has been convincingly repudiated by Legendre (1925). 



One can state, however, that, whereas some worms, 

 like Tubifex, exhibit very great resistance, other groups, 

 notably many polychaetes, support only a slight lack of 

 oxygen. The latter succumb to asphyxiation in closed 

 containers while varying amounts of oxygen still remain 

 in the water. Data concerning this point are presented 

 by Bounhiol (1902) and von Brand (192.7). The forms 

 involved are probably seldom, if ever, subjected to strictly 

 anaerobic conditions in nature. 



It is clear that the resistance to lack of oxygen depends 

 in many cases on the physiological state of the ivorms. 

 Any condition lowering the energy requirements will tend 

 to increase the anaerobic resistance. The less active the 

 anaerobic metabolism that is necessary to sustain life, 

 the less non-oxidized substances will accumulate in the 



