Jennings, Behavior of Paramecium. 473 



<iirection. This condition is apparently found in a number of 

 specimens in any given culture. 



2. Reaction to Gravity ; Gcotaxis. — The general features 

 of the reaction of Paramecium to gravity have been described 

 by Jensen (1893). Jensen further proposed a theory to ac- 

 count for the reactions ; but at the time his work was done, the 

 "action system" — the general complex of structural relations, 

 movements and reactions, by which most of the behavior is 

 brought about — was not known. Jensen's theory could there- 

 fore take no account of this system, and I believe that in view 

 of the known facts and of those which I shall bring forth in the 

 present account, it can be no longer maintained. My present 

 purpose is to describe the method by which the reaction to 

 gravity occurs, and to show the relation of this to other reac- 

 tions and to the "action system" of Paramecium. 



The gross facts are as follows : When Paramecia are 

 placed in a vertical tube, fairly free from other sources of stim- 

 uli, they swim upward, to the upper end of, the tube. Control 

 experiments show that gravity is the real directive influence. 

 But usually some individuals in any culture show the opposite 

 effect, swimming downward, while others do not become oriented 

 at all. In certain cultures the majority of the individuals 

 swim downward, or are indifferent. The reaction to gravity is 

 easily overcome or modified by the action of other agents (Sos- 

 NowsKi, 1899, Moore, 1903). 



Jensen's theory to account for the reaction to gravity was 

 as follows : The cause of the reaction is the difference in 

 pressure upon the two sides or ends of the animal ; the lower 

 end or side is in a region of greater pressure than the upper. 

 The greater pressure acts as a stimulus to cause the cilia on the 

 lower side of the body to beat more strongly. As a result, 

 the anterior end must be turned in the opposite direction (that 

 is, upward), until it points in the direction of least pressure. 

 The two sides are now similarly affected by the pressure, so that 

 there is no cause for further turning. Jensen's theory is thus an 

 application oi the typical tropism schema to the reaction to 



