168 



Gust. Alsterberg 



c) That the current of water produced by the rhythm can be most easily ex- 

 plained as a functional phenomenon for the supply of the enterorespiratory appa- 

 ratus with water out of the respiratory layer. 



d) That changes of length coincident with the rhythm, the purpose of which 

 has proved to be the augmentation of the effect of the latter, should also have the 

 function of increasing the respiratory area, a result that would have been easier to 

 attain by pumping water through a wide tubular structure, involves a new, compli- 

 cated hypothesis that is yet to be proved. 



e) That the enterorespiratory apparatus seems to enter into a state of dyspnoea, 

 which indicates that also this apparatus is capable of adapting itself to a large 

 extent. 



f) That in the light of the hypothesis that the enterorespiratory apparatus is 

 the only, or at least the principal, mechanism for supplying the demand for 0 2 , 

 the respiratory curve is capable of being readily interpreted without complicated 

 assumptions. 



3) That with respect to Bohn's grouping (1906) of the respiratory mechanism 

 of the Polychœtœ, the Tubificidce form a third collateral group, characterized by the 

 fact that the water is not pumped through any tubular structures by the respiratory 

 movements of the animals, but only to the distal section of the body by sinuous 

 oscillations. 



4) That under cecological conditions in nature, the following factors are of 

 importance: 



a) That respecting the rise of the respiratory layers above the mud, we must 

 also reckon with the tjrne as an important variable. In view of this factor the 

 respiratory layer will eventually attain a position of stability, situated nearer the 

 mud-level when 0 2 is present in high general concentration, and higher up, on the 

 other hand, when the concentration is lower. These proportions are also illustrated 

 by the curve for the ascent of the 0 2 -layers. 



b) That when 0 2 benefits the bottom organisms, it is always from the micro- 

 layer, whereas, immediately it is to be found in the macro-layer, it is present in 

 quite too small concentrations for the organisms in question. 



I. Respecting the 0 2 concentration of the respiratory layer, I have only a 

 few characteristics to set forth, namely, 



1) That its 0 2 concentration is not always the same, but fluctuates around an 

 optimum with maximum and minimum limits. 



2) Optimum represents the 0 2 "concentration to which the animal has the 

 greatest possibility of adapting itself and consequently can most easily do so. This 

 optimum lies presumably in the knee of the curve coinciding with an 0 2 concentra- 

 tion of about 2 — 3 c. c. 0 2 per litre. The facility with which adaptation to this 

 concentration takes place is characterized by irregularities in the commencement of 

 the rhythm, i. e. the option to follow the respiratory layer directly or indirectly. 



