CHAPTER X 



CONCERNING THE QUESTION OF ORIENTATION IN MOL- 



LUSKS, ARTHROPODS AND VERTEBRATES WITH 



SPECIAL REFERENCE TO CIRCUS MOVEMENTS 



AND THEIR BEARING ON THIS QUESTION 



I. General Account of Orientation 



We have found in our work on the lower metazoa that 

 as the organisms become more complex and the structures 

 more highly differentiated, the power of differential response 

 to localized stimulation by light becomes more highly 

 developed and plays an increasingly more important part 

 in orientation; and trial and random movements become 

 of relatively less importance. In the mollusks, arthropods 

 and vertebrates, there is little evidence of preliminary trial 

 movements in the process of orientation in light. If the 

 ray direction is changed these forms turn directly until 

 their direction of motion bears the same relation to the 

 source of light it previously had. I found this method of 

 orientation to hold in the following forms: Limnea columella, 

 Cyprls sp. (?), Daphnia sp.(?), Scapholeberisarmata, zoeae, 

 several species belonging to the Anomura and Brachyura, 

 Caprella sp. (?) and Bufo americanus. The work of Cole, 

 Bohn, Parker, Holmes, Yerkes, Harper, Hadley, Torelle 

 Radl and others shows the same to be true for numerous 

 other species belonging to these groups. 



The fact that these forms orient in light without prelimi- 

 nary movements does not however indicate the absence of 

 trial movements when subjected to other stimuli and is no 

 argument against the " trial and error " hypothesis in gene- 

 ral as Bohn (1908, pp. 77, 82) seems to Imply; nor does it 

 show how light acts as a stimulus. It probably means that 

 with reference to stimulation by light the power of differen- 



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