Jennings, Behavior of Paramecium. /^yj 



mecia first swim vertically upward against the inclined wall, then 

 turn away, and again swim vertically up till they strike it, etc., 

 in other cases they swim obliquely upward along the wall. 

 From this latter fact he concluded that they swim in the direc- 

 tion of decrease of pressure, instead of in the direction of ac- 

 tion of gravity. It is difficult to imagine from what data or by 

 what process of reasoning this conclusion was reached. The 

 decrease in pressure of course takes place in an inclined tube 

 in the same direction as in a perpendicular one, and coincides 

 in both cases with the direction of gravity. Jensen's experi- 

 ment was not of the least value in differentiating the two direc- 

 tions ; indeed, so long as the pressure is due to gravity the two 

 directions in question must coincide. If, therefore, the obser- 

 vations mentioned speak in the least against the view that the 

 organisms tends to move in the lines of the direction of gravity 

 (which, as Davenport, 1897, p. 123, has shown, they do not), 

 then they speak equally against the view that the movement is 

 in the direction of decrease of pressure. 



What then is the effective stimulus in the reaction to grav- 

 ity ? In the other reactions of Paramecium we have found that 

 the effective stimulus is due to some change in the conditions, 

 or, what amounts to the same thing, in the relation of the or- 

 ganism to the conditions. In the reactions to gravity exactly 

 the conditions are present for the production of such changes, 

 and the reaction is of precisely the character that might be ex- 

 pected from such changes as occur. The conditions are quite 

 parallel to those found in the reactions to water currents. The 

 changes in question are brought about through the fact that 

 Paramecium swims in a spiral, swinging successively in many 

 directions. In an unoriented specimen the upward phase of 

 the swerving is resisted by gravity, making the motion more 

 difficult ; the downward phase is assisted, making the motion 

 easier. The effect of these repeated changes in resistance or 

 the ease of swimming is similar to the effect of repeated streams 

 of water directed on a quiet animal. The result of such en- 

 vironmental changes is, as we know, to produce the "avoiding 

 reaction," and this is what we see in the reaction to gravity. 



