THE ROLE OF RANDOM MOVEMENTS IN THE 

 ORIENTATION OF PORCELLIO SCABER 



TO LIGHT 



HARRY BEAL TORREY and GRACE P. HAYS 



Reed College, Portland, Oregon 



1 



In his admirable paper on " The Selection of Random Move- 

 ments as a Factor in Phototaxis," Holmes 1 has given great 

 significance to random, that is, spontaneous, non-directive 

 movements in the orientation of earthworms, blow fly larvae, 

 and leeches, to light. As he carefully watched the movements 

 of these organisms under the influence of light, it " soon devel- 

 oped that what seemed at first a forced orientation, the result 

 of a direct reflex response, is not really such, but that the orien- 

 tation which occurs and which is often quite definite is brought 

 about in a more indirect manner by a mode of procedure which 

 is in some respects similar to the method of trial and error fol- 

 lowed by higher forms." The organism becomes oriented by 

 following up those random movements which bring them away 

 from the source of light. 



While our experiments on the larvae of an undetermined 

 species of blow fly and on a species of earthworm (Allolobophora 

 sp.) materially lessen for us the importance of random move- 

 ments as a factor in the orientation of these organisms to light, 

 our conclusions are in complete accord with Holmes' view that 

 the type of reaction he describes " differs from Jennings' ' motor 

 reflex ' by which many of the so-called tropic reactions are 

 produced in the Protozoa." This difference has little signifi- 

 cance for Mast 2 who believes that " the only difference between 

 the orienting reactions in the two classes of animals mentioned 

 is that the unicellular forms studied by Jennings always turn 

 toward a structurally defined side, while the metazoa investi- 



Journal of Comparative Neurology and Psychology, 1905, No. 15, p. 18. 

 Light and the Behavior of Organisms, 1910, p. 51. 



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