THE HELIOTROPISM OF ONCHIDIUM: A PROBLEM IN 

 THE ANALYSIS OF ANIMAL CONDUCT. 



By W. J. CROZIER and L. B. AREY. 



{Contributions from the Bermuda Biologual Station for Research, No. 108, and from 

 the Physiological Laboratory, College of Medicine, University of Illinois.) 



(Received for puDlication, July 21, 1919.) 

 I. 



Onchidium floridanum is a small naked puLmonate which lives in holes 

 and crevices of the shore between tides. A description of the bionomics 

 of this mollusk we shall present more fully in another connection/ 

 omitting here those aspects not bearing directly upon the most curious 

 puzzle propounded by its behavior, which in brief is this: under 

 natural conditions the movements of Onchidium take place in the light, 

 but without reference to photic control; yet when the animal is tested 

 apart from its natural surroundings it is found always to be negatively 

 phototropic. 



In view of this apparent incompatibihty, in reality very deep 

 seated, one is led to inquire: Is the behavior of Onchidium an instance 

 where ''unnatural conditions" imposed in phototropism experiments 

 vitiate attempted analytic explanation of normal activities and 

 more especially what, fundamentally, is the sense of the state of 

 affairs we have outHned, in terms of adaptation? 



II. 



1. Phototropism. — Exploration of the dome-shaped dorsum of the 

 mantle of Onchidium with a light-beam 0.3 mm. in diameter and of 

 moderate intensity showed that the movements of the mollusk, in 

 the laboratory could be directed at will by even as small an illumi- 



^ A preliminary statement of certain of the results, utilized for different pur- 

 poses than in the present connection, is found in the papers: Arey and Crozier 

 Cf5 (1918) and Crozier and Arey (1919a). 



a^ 107 



