RESPIRATION OF DYTISCUS MARGINALIS L. 341 



E. In the second place one can regularly observe very distinct 

 hydrostatic movements. When the animal moves up and down 

 in the bottle the air in the reservoir is alternately expanded and 

 compressed. The extreme sensitiveness of the apparatus is shown 

 by the fact that even these minute changes in volume can be 

 read in the capillary tube. This gives, in the second place, a 

 good test as to whether or not the apparatus is working all right 

 in a certain series of experiments. Sometimes these movements 

 do not show because the cleft is completely closed. 



F. Finally, the definite proof can in this way be given that the 

 animal breathes under water. This fact has been denied by 

 several authors. Immediately after diving the animal is usually 

 very motile. After some time, however, it comes to rest and 

 may sit quietly at the bottom of the vessel for a while. Then 

 one can observe several times a very distinct moving up and down 

 of the droplet: in the meantime the gas-bubble in the cleft of 

 the abdomen makes synchronous movements. The latter move- 

 ments have already been observed by some authors, while others 

 did not seem to realize their importance. 



A diagrammatic representation of the whole process is tried 

 in figure 2. The movements of the droplet are supposed to be 

 projected on a vertically moving plane. In that way the ordinate 

 gives the time, while from the abscissa one can read the location 

 of the droplet at every moment. 



We must now try to give an adequate explanation of these 

 facts, and in doing so we must keep in mind the following 

 considerations : 



1. It has been shown by Ege^ that the oxygen of the reservoir 

 is consumed during the animal's stay in the water. I could prove 

 the exactness of this statement by the following analyses, which 

 have been made with Krogh's apparatus for micro-gas analysis:^ 



^ Aug. Krogh. On micro-analysis of gases. Scand. Arch. f. Physiol., Bd. 

 20, 1908. 



