170 THE ELEMENTAEY NERVOUS SYSTEM 



of a physiological turn, an explanation of the modified 

 form of response is found in an understanding of the 

 interaction of the second stimulus and the receptor, to- 

 gether with the chain of events that terminate in the 

 muscular movement. This form of explanation is con- 

 cerned exclusively with the working mechanism as such 

 and has nothing to do with its historical origin. The 

 second form of explanation, the one more likely to be 

 adopted by those of a more biological bent, seeks for an 

 understanding of the modified response in the influences 

 that have emanated from the original response and thus 

 bring that response into historical relation with the sec- 

 ond and modified one. This form of explanation em- 

 phasizes the effect of the history of the animal on its 

 immediate state. Both forms of explanation depend, of 

 course, upon material changes that occur within the ac- 

 tinian, but in the first attention is directed especially to 

 those changes that take place in the course of the modi- 

 fied response, whereas in the second emphasis is put on 

 the changes that link the first response with the second. 

 The opinion that the past history of an individual 

 actinian is a potent factor in understanding its behavior 

 has been expressed not only by Jennings (1905) but also 

 by Pieron (1906 c), who declared that in this respect the 

 responses of actinians included traces of those activities 

 characteristic of the central nervous organs of higher 

 animals. But very little work has been done on actinians 

 to ascertain the extent to which such central activities as 

 those just indicated may extend. The limited range of 

 response in these animals restricts such experimentation 

 considerably. Heretofore associative processes have 

 never been directly identified in actinians and repeated 

 efforts in this direction have always yielded negative re- 



