Die respiratorischen Mechanismen der Tubificiden 169 



3) The infraoptinial 0 2 concentrations are represented by the dyspiicea-leg of 

 the respiratory curve, which shows how the animal is able to follow, with increasing- 

 difficulty, the respiratory layer in its upward wanderings. In doing this the animal 

 adjusts itself to an ever declining concentration of 0 2 , e. g. by increased intestinal 

 respiration, and finally the poorest layer in respect of 0 2 comes, and this succeeds 

 in prompting the animal to reaction against maximum dyspnoea. Maximum dysp- 

 noea (the minimum limit) is to be sought alter when the general 0 2 concentration 

 is at 0"O8 c. c. per litre, and this should correspond fairly nearly witli that of the 

 »bottom -water». 



4) The supraoptimal 0 2 concentrations are represented by the apncea-leg of 

 the respiratory curve, which shows how the animal succeeds, under ever increasing 

 difficulties, in evading the effect of such supraoptimal concentrations. Presumably, 

 the upper maximum limit should represent the circumstance that the adaptive- 

 apparatus did not concede the animals any further facilities for evading still higher 

 concentrations of 0 2 . I have found that when apnoea supervenes the animals do 

 not avoid a layer with a concentration of 7 c. c. 0 2 per litre. Probably, however, 

 the hunt is not yet reached, but it must be near. 



5) The aerobic respiratory mechanism in the Tubifex and Limnodrilus is thus 

 adjusted in conformity with the euryoligoxybiontic (Fehlmann 1917) principle. 



J. Effect of C0 2 . 



1) C0 2 causes a reflexive suspension of the rhythm, which recovers in time. 



2) It lowers the frequency of the returning rhythm, especially at times of 

 rather high concentration, to values far below the minimum values of the 0 2 ex- 

 periments, and if the 0 2 concentration is sufficiently high, no rhythm at all occurs, 

 i. e. the cessation just referred to has developed into a persisting paralysis. 



3) It affects the length of the tail, though not in proportion to the concentra- 

 tion of C0 2 , but in proportion to the cessation of the rhythm. The change of 

 length probably falls within the bounds of the compensatory value of the rhythm, the 

 latter being primarily determined by the concentration of 0 2 present at the moment. 



4) It retards the rhythm in the respiratory curve when it occurs in high con- 

 centrations, but otherwise the curve is not affected. It is therefore obvious that 

 C0 2 has nothing to do with the essential shaping of this curve. 



5) That it has nothing to do with the origin of the rhythm as a causal agent, 

 I consider to be obvious from the fact that rhythm goes on even under very high 

 concentrations of C0 2 . Newly produced C0 2 originating from the work of the cells 

 can now have little significance in relation to the whole. 



K. Effect of Temperature. 



1) The frequency is altered in conformity with Van't Hofp's rule (the RGT- 

 Rule) with a temperature quotient = 2 i. 



2) The lowest value of the frequency falls below its lowest value under super 

 abundance of 0 2 , and the highest value is much above the highest under a defici- 

 ency of O g . 



22 



