W. J, CROZIER AND L. B. AREY HI 



of this animal to come out into the sunHght; it must do so in order to 

 obtain food; therefore, it disregards the dictates of its negative pho- 

 totropism. This notion is sufficiently disposed of in the foregoing 

 section; the negative heHotropism of Onchidium is not "disregarded" 

 normally, it is centrally inhibited by other impulses competing for the 

 nervous control of the body musculature. The further question re- 

 mains: What is the role of the heliotropism of Onchidium} The mere 

 existence of this type of response is sufiSciently accounted for by the 

 composition of the animal and the structural disposition of its parts. 

 We have to inquire if this heHotropism is ever of "use." 



Note in the first place our finding that the movements of an 

 Onchidium on a rock substratum remote from its own "nest" are 

 largely, and, in many instances, exclusively determined by the position 

 of the sun. Might it not then be supposed that if an Onchidium 

 should accidentally suffer displacement from its usual surroundings, 

 its negative orientation by light would force it to enter some crevice 

 or rock cavity, thus securing shelter and perhaps membership in a 

 new colony? But the facts are different. The natural displacement of 

 an Onchidium in this way is probably of very rare occurrence ; these 

 snails possess a well disposed system of repugnatorial glands (Crozier 

 and Arey, 1919a), which makes such an event unlikely. Much more 

 serious, however, is the fact that the animal withdraws sharply when 

 shaded, as already stated. Furthermore, numerous instances we have 

 carefully watched have yielded not one good case in which an On- 

 chidium would enter a strange ''nest" and remain there for more than 

 a few minutes near the entrance (Arey and Crozier, 1918) ; they always 

 crept out and away. These two kinds of response make it improb- 

 able that a ''lost" Onchidium would seek refuge in a strange de- 

 pression, and those which were purposely displaced in this way 

 remained in fact creeping about on the rock until washed off by the 

 rising tide. 



IV. 



We must refuse, then, to admit any obvious adaptive significance 

 inhering in the photic behavior of Onchidium. Loeb has pointed 

 out (1916) that positive heHotropism occurs in several arthropods 

 "which have no opportunity to make use of it." Instances of this 



