145 



the density of organisms around the drop and may give 

 rise to a ring formation around a high concentration of 

 HC1 although the organisms are not positive to the acid. 

 Jennings found, however, that when such organisms are 

 in a drop of weak acids which do not paralyze the organ- 

 isms quickly, e.g., 1/50 per cent, acetic or in C0 2 solutions, 

 they become negative to the surrounding neutral medium 

 (H 2 or hay infusion) and stay in the acid. He, therefore, 

 assumes that the organisms are positive to weak acid, and 



Fm. 41. Reaction of Chilomonas to a drop of 1/50 per cent. HC1. a, preparation 

 immediately after the introduction of the drop (no organisms either within or gathered 

 about the drop), b, the same preparation a few minutes later. (After Jennings.) 



negative to strong acid as well as to their natural neutral 

 or faintly alkaline medium. 



This negativity to their natural surroundings when 

 in weak acid as well as to strong acid when in weak acid 

 Jennings does not interpret in terms of the tropism theory, 

 and in this he is probably correct. He interprets both 

 phenomena as a trap action due to the asymmetry of 

 certain infusorians ; a sudden change in the concentration 

 of a solution causes a reverse of the stroke of their cilia 

 by which the organism is driven back. When the old nor- 

 mal stroke of the cilia is resumed the direction of the 

 locomotion is changed on account of the asymmetrical 

 arrangement of the cilia. This happens when the organ- 

 isms go from weak into strong acid or from weak acid into 

 10 



