330 



Scientific Proceedings (123). 



155 (1902) 



Symmetry of heliotropic orientation in slugs. 



By W. J. CROZIER and W. H. COLE. 



[From the Zoological Laboratories , Rutgers College, New Brunswick, 

 N. J., and Lake Forest College, Lake Forest, III] 



The creeping of the slug Limax maximus shows no apparent 

 neuromuscular asymmetry. But in heliotropic orientation there 

 is chiefly involved the parietal musculature, where differential ac- 

 tivity might conceivably be associated with the fundamental tor- 

 sion of the body of the gasteropod. The point has been studied 

 in relation to the occurence of "trial movements" at the beginning 

 of the act of orientation, and has a special significance also for the 

 analysis of "circus movements." 



Experiment shows that a previously quiescent, dark-adapted 

 Limax, illuminated from in front, turns with equal frequency to 

 the right or to the left. Limax is negatively heliotropic, for all 

 intensities used in these experiments. A similar equal frequency 

 of turns to right and to left is found under non-stimulating (red) 

 light. It would be incorrect to speak of these movements as 

 "trial movements," for in a series of 142 observations it was found 

 that in 122 the direction of orientation coincided with that of the 

 first indication of turning; only in 20 did the side definitively con- 

 tracted differ from that first contracted (8 right; 12 left). 



The relative tensions developed by the two sides of the body, 

 when right or left is the particular side toward which orientation 

 is made, may be decided by comparing the rates of orientation in 

 the two cases. Making allowance for the fact that the rate of 

 orientation varies inversely with the size of individual, by com- 

 paring only "right" and "left" records for the same individuals, 

 it is found that there is no sensible muscular asymmetry to be 

 considered in the measurement of circus movements. Asymme- 

 try of this type reported for some shelled gasteropods may be due 

 to the mechanical effect of the shell's weight. 



