MOVEMENTS OF AQUATIC MICRO-ORGANISMS 305 



their movements allows this action of gravity to be brought 

 into play." x 



" If there are few Euglenae present in the liquid they may 

 turn into a more or less horizontal position in contact with the 

 surface film or, if the vessel is closed, with the solid body, cork, 

 glass plate, etc., which may be used for that purpose. This 

 contact reaction interferes, as Loeb 2 points out in the case of 

 Paramecium, with the reaction to gravity. So long as the 

 Euglenae remain in this position, the action ol gravity may be 

 inhibited But if they are disturbed in any way, as by agitating 

 the surface of the liquid with a glass rod or by shaking the 

 vessel, they become once more oriented into a vertical position 

 and if a sufficient number has accumulated they may sink again. 

 This, no doubt, partly explains the disappearance of the motile 

 Euglenae from the surface of water which is observed in nature 

 during rain or wind." 3 



We are now in a position to answer the first of the questions 

 proposed on p. 299 by saying that both the upward and down- 

 ward movements of Euglenae are set up by gravity, modified 

 in some cases by crowding ; and further, that the action of 

 gravity appears to be mechanical, involving no physiological 

 response on the part of the organism. 



Let us next consider the arrangement of Euglenae into net- 

 works, neglecting the fact that they may at times be resolved 

 into more or less separated groups. At first I thought it 

 possible to explain the networks by differences in the food- 

 supply or in the amount of carbon and dioxide and oxygen 

 present. 



" Euglenae are certainly attracted to some extent by oxygen, 

 as Aderhold 4 has shown; they tend to accumulate around 

 bubbles of air in the water and their accumulation at the free 

 surface of water in contact with air is, no doubt, partly due to 

 this. I found, however, that the aggregation takes place just 

 as well and as quickly in water which has been thoroughly 

 boiled to get rid of the air as in water that has been well aerated, 

 so that it could not be due to variation in the oxygen contained 

 in the water. Nor is it due to air at the surface, for the aggre- 

 gation is as perfect in closed vessels from which all air is 

 excluded as in open vessels exposed to the air. That the 



1 Wager, loc. cit., p. 370. 



1 Loeb, J., The Dynamics of Living Matter, New York, 1906. 



3 Wager, loc. cit., p. 371. 



4 Aderhold, R., "Beitrag zur Kenntniss richtender Krafte bei der Bewegung 

 niederer Organismen," Jenaische Zeitschrift /. Naturwiss. 22,310, 1888. 



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