78 BEHAVIOR OF THE LOWER ORGANISMS 



C. Reaction to Centrifugal Force 



Conditions similar to those due to gravity may be produced by a 

 centrifugal force, and Paramecia then react, as might be expected, in 

 the same way as to gravity. Jensen (1893) shows that if a tube contain- 

 ing Paramecia is placed in a horizontal position on a centrifuge and 

 whirled at a certain rate, the infusoria tend to swim toward that 

 end of the tube next to the centre. In a tube 12 cm. long, with the 

 inner end 2 cm. from the centre, the phenomena were well shown 

 when the tube was whirled at the rate of four turns per second, for ten or 

 fifteen minutes. In such a tube the Paramecia at the outer end, where 

 the movement is fastest, are carried by the centrifugal force, against 

 their active efforts, to the outer end of the tube ; this is of course a purely 

 passive phenomenon. The remainder of the Paramecia swim toward 

 the end of the tube next the centre and collect there ; this is the active 

 part of the reaction. 



This movement toward the inner end of the tube is doubtless due to 

 the same causes, whatever they may be, that produce the upward move- 

 ment in the reaction to gravity. Lyon (1905) has shown that the body 

 contains substances of varying specific gravity, some of which collect, 

 under strong centrifugation, at that end of the animal which is at the 

 outer end of the tube. This redistribution is probably the cause of the 

 reaction to centrifugal force. If the passage of such substances into 

 the anterior end should act as a stimulus to the usual reaction, this 

 would produce the results actually observed. 



6. RELATION OF THE ORIENTATION REACTIONS TO OTHER REACTIONS 



We are now in a position to define the difference between these orien- 

 tation reactions and the others that we have described, and to see why 

 the result of the avoiding reaction is to produce a certain position of 

 the body axis in one set of cases, while it does not in the others. 



In the reactions to mechanical stimuli, chemicals, osmotic pressure, 

 heat and cold, and powerful light, the avoiding reaction is caused by the 

 transition from one external condition to another; by a change in the 

 intensity of action of some agent, — the change being of such a charac- 

 ter as to lead away from the optimum. As a result, the organism tries 

 repeated different directions of movement (in the avoiding reaction) till 

 it hits upon one in which the transition is toward the optimum instead 

 of away from it ; in this direction it continues. This does not require 

 the body axis to take any definite orientation, since as a rule there are 

 various directions in which the animal can move and be on the whole 

 approaching the optimum. Furthermore, the body axis might be in any 

 position, provided the movement were on the whole toward the optimum. 



