THE ORIENTATION OF PORCELLIO TO LIGHT 111 



gated by Holmes are not thus limited in their direction of 

 turning." 



In thus minimizing a difference to which Holmes has explic- 

 itly called attention, Mast may have missed a cardinal point 

 in Holmes' illuminating discussion. The direction of the random 

 movements of the blow fly larvae as observed by Holmes is 

 not predictable so far as it bears no definite relation to the source 

 of light. The direction of the movements of Euglena, an organ- 

 ism in which the 'motor reflex' plays an important part in 

 its orientation to light, is predictable, since it does bear a defi 

 nite relation to the source of light. In the one case, the orient- 

 ing movements, made at random, are not controlled, as to 

 direction, by the light; in the other case, the orienting move- 

 ments are definitely controlled, as to direction, by the light. 

 In the former, selection operates among so-called trial move- 

 ments; in the latter, in so far as the movements are controlled 

 or forced by an external agency, the method of trial is excluded. 

 This difference, then, is of no little significance in an attempt 

 to determine — as this paper is attempting to determine for 

 certain organisms — the actual value of the orientation hypo- 

 thesis that rests upon the assumption of trial movements. 



The fact that some authors do not distinguish between ran- 

 dom movements and directive movements forced by the en- 

 vironment has been a source, of some confusion in the literature 

 of animal behavior. Further confusion has centered about the 

 conception of symmetrical stimulation repeatedly emphasized by 

 Loeb and recently reaffirmed by Parker. 3 Investigators of the 

 orienting reactions of non-symmetrical protozoa or symmetrical 

 organisms such as rotifers and worm larvae that swim, like the 

 protozoa, in spiral courses, have had difficulty in seeing the 

 applicability of this conception to their material. That the 

 conception is applicable, however, to the behavior of such organ- 

 isms as Euglena, though not in the form apparently anticipated 

 by some of its critics 4 a recent paper 5 has attempted to show. 

 And its applicability to the orientation of earthworms and blow 



Journal of Animal Behavior, 1911, No. I, p. 461. 



4 Mast, 1910, p. 85. 



Torrey, Science, No. 38, p. 873. 



