582 ^.'Journal\)f Comparative Neurology and Psychology. 



and eighteen cases of creeping backward (type III). Using the same 

 apparatus so as to give the maximum sudden ilhimination of the 

 anterior end, fifty trials gave four of type I, five of type II and 

 forty-one cases of backward creeping. Seven of the latter showed 

 initial retraction of the head, in some cases strongly marked. The 

 results indicate the infrequency of shrinking movements before nega- 

 tive turning and hence the unimportance of type II ; also the absence 

 of shrinking before backward crawling in light of moderate strength. 

 The long latent period and absence of shrinking point to such reac- 

 tions as constant stimulus effects, for responses attributed to the 

 change produced by the stimulus ought to give evidence of the shock 

 in resulting movements or inhibitions. Withdrawal into the bur- 

 row may be effected by backward creeping without evidence of shock 

 and hence the analogy with Serpula fails in some cases, as the latter 

 is described as suddenly withdrawing into its tube on the stimulus 

 of a shadow. Perichseta may, however, with strong light and sudden 

 illumination give a shock reaction analogous to that of Serpula. 



If illumination of the anterior end is continued through the period 

 of backward creej)ing, the worm eventually resumes forward crawl- 

 ing and turns away from the light. Or if the illumination is shifted 

 from the anterior to the posterior end during backward crawling, the 

 worm moves forward and almost always turns away from the previ- 

 ously stimulated side, showing the after effects of the stimulus. 

 These after effects show the tonic character of the stimulus very 

 clearly. During backward crawling the effect of the stimulus on 

 the anterior end was latent, though it may, to be sure, sometimes be 

 exhibited in uncoordinated turning movements of the head. 



In weak light the worm makes extension movements of the head, 

 thrusting it forward and drawing it back. Such retractions might 

 be regarded as due to shock, if the worm is more sensitive when 

 extended. But they have more of the appearance of a return to 

 normal after extreme extension than of shock retractions. After 

 feeling about, the worm usually turns decidedly in one direction. 

 Torrey ('07) referred to a paper by the writer on Perichseta as 

 affording an instance of the distinction between unterschiedsemp- 

 findlich and tropic responses. 



