Jennings, Behavior of Paramecium. 479 



The stimulus induced by the variations in the resistance 

 due to gravity is of course a very light one. and observation 

 shows that it is easily modified or masked by other stimuli. 

 Chemical, mechanical and electrical stimuli overcome the reac- 

 tion to gravity, hence the necessity of having the Paramecia in 

 nearly pure water and in a clean tube if the reactions to gravity 

 are to be seen clearly. If these conditions are not fulfilled, the 

 Paramecia may collect in any part of the tube, through reac- 

 tions to chemical stimuli, and to contact with solids. It may 

 perhaps be said in general that the reaction to gravity shows 

 itself only when the animal is not subjected to other effective 

 stimuli. 



Jensen (/.' c.') showed that when placed on the centrifuge 

 Paramecium reacts with regard to the direction of the centri- 

 fugal force in the same way as to gravity. The animals orient 

 themselves and swim in the direction opposite to that in which 

 the centrifugal force tends to carry them. In these experiments 

 the conditions are of course present for the same sort of reac- 

 tions that we find under the action of water currents and of 

 gravity. In one phase of the spiral course the movement of 

 the unoriented animal is assisted by the centrifugal force, in an- 

 other resisted ; the changes thus produced lead to reaction and 

 orientation in the way already described. 



Summary. — The reactions to water currents ("rheotaxis"), 

 to gravity ("geota.xis") and to centrifugal force are in Parame- 

 cium essentiall}' the same, and due to similar conditions ; they 

 may be summed up as follows : The unoriented individual is 

 subjected, owing to its spiral course, to repeated changes of 

 pressure and of the resistance to its movements ; in one phase 

 of the spiral the motion is assisted, in another resisted. These 

 changes induce the usual reaction ; through the consequent in- 

 creased swerving toward the aboral side, with the revolution on 

 the long axis, the animal occupies successively many different 

 positions, till one is found in which these changes no longer 

 occur, when there is no further cause for reaction. Such a po- 

 sition, in which the relation of the movement to the resistance 

 remains constant, is found only in orientation with the axis of 



