H. MTJNRO FOX 503 



to reduced oxygen concentration that the organisms are attracted 

 both in the case of the carbon dioxide bubble and of the hydrogen 

 bubble. Part of the oxygen in solution in the water diffuses into the 

 gas bubble, where the partial pressure of oxygen is nil, and so a region 

 of reduced oxygen concentration is established in the water around 

 the bubble. The flagellates are attracted into this region just as 

 they are attracted into a central region under the cover-glass where 

 the oxygen concentration is lowered by their own respiration. Just 

 as in the latter case the aggregation occurs sooner in a non-aerated 

 than in an aerated preparation, so the response to a hydrogen bub- 

 ble is found to be greater in a non-aerated suspension. 



The flagellates collect immediately around a hydrogen bubble but 

 the intensity of the aggregation diminishes with time, until after a 

 certain interval there are no more Bodo around the bubble than in 

 the general suspension. This is because the bubble has now absorbed 

 from the water enough oxygen to be in equilibrium with the dis- 

 solved oxygen and there is no longer a region of low oxygen concen- 

 tration in the water around the bubble. After this the continually 

 decreasing oxygen tension in the water, due to the respiration of the 

 flagellates, falls below that in the bubble and the flagellates move 

 away from the latter which is now giving back oxygen to the water. 

 The center of the preparation is then cleared of Bodo, the peripheral 

 band formed in the usual way, and in this process the bubble acts 

 as an air bubble, becoming surrounded by an inner ring of flagellates 

 which gradually approaches it. 



Thus the experiments with acids and with gas bubbles not only 

 dispose of the view that spontaneous aggregations are due to positive 

 chemotropism to acid produced by the respiration of the flagellates 

 but they also strengthen the explanation of the phenomenon given 

 above. 



The Effects of Excess and of Absence of Oxygen. 



There is a relation between the oxygen content of the Bodo culture 

 media and the preference shown by the flagellates for an optimum 

 oxygen concentration lower than the saturation concentration of the 

 gas in water in contact with air. For a series of determinations 

 showed that, in the grass infusions contained in upright unstoppered 



