Jennings, Behavior of Paranieciuni. 467 



in the reactions is the direction of movement of the animal, not 

 the mere orientation, as has sometimes been held. The signifi- 

 cance of these relations in connection with the theory of gen- 

 eral "pain reactions" I have considered elsewhere (Jennings, 

 1904, U). Here we may point out, as a relation of some in- 

 terest, that in Paramecium it is an injurious or negative stimulus 

 that primarily induces motor reactions. This is not at all in 

 agreement with the theory sometimes set forth, that the effect 

 of such stimuli is to cause a cessation of activity. 



In no case, so far as I am aware, has it been shown that 

 the reaction in Paramecium is due to the difference in intensity 

 of a graduated stimulus on the two sides or ends of the animal, 

 as is assumed by the orthodox tropism theory. In most cases 

 it has been demonstrated that the determining features of the 

 reaction are not of this character. 



I have above illustrated the fact that in reactions to chem- 

 icals and in temperature reactions, it is a change that causes the 

 response ; details are given in my previous papers. In the re- 

 actions to changes in osmotic pressure, a very marked change 

 to a higher pressure is required to produce reaction ; the oppo- 

 site change, even to distilled water, is without effect. In the 

 reaction to mechanical stimulation, sudden contact of the anter- 

 ior end with a solid produces the reaction, though continuous 

 contact is of no effect. Paramecium is not, so far as known, 

 sensitive to light. But in other infusoria the writer has recently 

 shown (Jennings, 1904) that it is the change in light intensity, 

 at the sensitive anterior end, that induces reaction. The reac- 

 tion occurs when the change is due to an actual alteration in the 

 source of light, or when it is due to a movement of the organ- 

 ism. Orientation is produced through the fact that in the spi- 

 ral course the anterior end of an unoriented organism is repeat- 

 edly subjected to changes in illumination. To these changes it 

 reacts, by the method of "trial and error," above described, till 

 it comes into a position where such changes no longer occur ; 

 such a position is found only when the animal is oriented. The 

 reactions to light are particularly instructive for the part played 

 by the spiral course, with its swerving from side to side, in 



