348 H. C. VAN DER HEYDE 



of air bubbles from the dorsal space would be explained — is dis- 

 proved by the fact illustrated in table 1 (decrease of oxygen till 

 about 3 per cent) and by our above-given calculations. 



e. Concluding these theoretical considerations, we may state: 

 1) that from the statistical standpoint a state of sliding equilib- 

 rium between bubble and water will be reached after a while 

 in which Oo diffuses in and compensates the outflow of N2 (and 

 CO2?) ; 2) taking into consideration the animal's consumption of 

 oxygen, we can see that this inflow has no real 'biological impor- 

 tance' in the sense in which Ege used this expression. 



This attempt at an explanation revealed some interesting facts, 

 but does not give the solution of our problem. It appears that 

 the problem cannot be solved by applying physical chemistry to 

 the processes which happen during the animal's stay in the water. 

 So we are logically obliged to find its solution in the process of 

 breathing itself and in doing so we must keep in mind: 1) that 

 the animal expels the droplet of petroleum — this proves that a 

 vacuum must be made somewhere as shown above; 2) that the 

 air in the dorsal space in the moment of diving is a mixture of 

 atmospheric and tracheal air (see 5). I believe that these facts 

 enable us to give only one explanation of the whole mechanism as 

 follows (though it took me a long time to find it — it is something 

 like the egg of Columbus and extremely simple .... 

 after one has realized it) : 



As soon as the animal has opened its abdominal cleft, it moves 

 its back slowly downwards. Consequently, the air in the stig- 

 mata is compressed just a little bit and flows out, partly through 

 the dorsal, partly through the terminal stigmata. In that way 

 no movement of the droplet can be expected because no vacuum 

 nor noticeable compression is made. In this connection it is 

 remarkable that in several experiments I could observe that the 

 droplet continued its backward movement for a few seconds 

 after the cleft had made communication between the dorsal space 

 and the atmosphere. This may perhaps be an indication in 

 favor of my hypothesis. The only effect of the postulated move- 

 ment is that the tracheal air is expelled from the tracheae into 

 the dorsal space and perhaps into the free air (in the latter case 



