208 



LESLIE B. AREY AND W. J. CROZIER 



surfaces inclined at any angle above 30° with the horizontal. 

 For these tests the larger animals are best. 



Observations in the field and many tests in the laboratory 

 show clearly that there is no tendency for Chiton to preserve a 

 constant dorsoventral orientation. In rock crevices they occur 

 'upside down' with great frequency. 



Careful observation of the movements of a chiton during geo- 

 tropic orientation affords a clue as to the nature of the determin- 

 ing stimulus. Accurate outlines of a specimen orienting in this 

 way are given in figure 12. Inspection of these outlines will 

 show that the sequence of events in orientation is as follows: 



IICSlRi5i.E riinz) 



C5l>^DLEL PRESSED To SLASs) 



Fig. 12 Outlines of the successive positions assumed by a Chiton in ori- 

 enting upward from a horizontal position, under water, on a vertical surface; 

 ventral aspect. (Traced through glass, on thin paper.) X f. 



The girdle becomes freed from the substratum, so that the ani- 

 mal remains attached by the foot only; when in the horizontal 

 position (fig. 12, II), the weight of the body causes it to fall 

 slightly, producing an uneven tension in the muscles, those on 

 the higher side being stretched. The animal swings until this 

 unilateral tension is relieved. It turns anterior end up, probably 

 because that end is the more sensitive. The tendency is for the 

 animal to turn toward the stretched side ; the tenser muscles are 

 the ones which contract. With animals in the vertical position 

 (fig. 12, V) the downward pull of the creature's weight is exerted 

 at the posterior end. 



