176 University of California Publications in Zoology. [Vol. 4 



nients of that part. Thus, movement in a certain direction in- 

 duces a state which leads to movement in another direction, etc.^'' 



2. While in this physiological state a localized stimulus acts 

 on the body. This may be any of the stimuli enumerated in pages 

 1^5-142: light coming from a certain direction, the touching of 

 certain rays to the underlying surface, etc., etc. This localized 

 stimulus is transmitted (evidently mainly by the nervous system) 

 to all parts of the body ; it produces a differential effect on the 

 different parts, depending on their position with reference to it. 

 The rays on one side are caused to twist, to send forth tube feet 

 in a certain direction, to attach, and to pull. Those of the other 

 side react differently, releasing, rising up and swinging over, etc., 

 while the intermediate rays are caused to act in a still different 

 way. The different effects produced in different parts depend 

 (1) on the past stimuli and reactions that have affected these 

 parts, and (2) on the localization of these parts with respect to 

 the source of stimulation. 



3. But, as we have seen, before the reaction has become 

 well started the localized stimulus may be removed, or it may be 

 localized in an entirely different way, yet the complex reaction 

 continues as before. This shows that the stimulus sets in progress 

 certain processes in the different parts of the body (depending on 

 its localization relative to each part), and that these processes, 

 once started, are not readily stopped; they continue even in spite 

 of the incidence of new stimuli, which would have started a new 

 set of processes if these had not been in progress. 



4. Throughout the progress of the differential reaction of the 

 different parts new intercurrent stimuli acting on the various 

 parts causes them to modify slightly their method of behavior 

 without altering the general trend of the processes of turning; 

 the tube-feet attach when they come in contact, etc., etc., so that 

 there is throughout an adaptation to the present conditions. 



5. As soon as the normal position is restored this restores 

 again the normal physiological condition that existed before the 

 inversion of the animal ; as a result all the processes involved in 

 the reaction cease. 



" Compare the account of these relations in the earthworm (Jennings, 

 1906 a). 



