112 HARRY BEAL TORREY AND GRACE P. HAYS 



fly larvae to light has been convincingly discussed by Parker in 

 the paper just mentioned. 



These conceptions of symmetrical stimulation and of forced 

 directive movements have long characterized the tropism hypo- 

 thesis, whatever other attributes it may be said to possess; and 

 they appear to be quite inconsistent with the conception of 

 orientation by the selection of trial reactions. There should be 

 little danger of confusion, then, in designating as tropic reac- 

 tions not only the very gradual turning movements that may 

 or may not be connected with tonic contractions accompanying 

 constant stimulation, but also the more abrupt and angular 

 turning movements composed of a series of forced shock reac- 

 tions, all in the same general direction, that we have repeatedly 

 observed' in the orientation of Euglena to light. Both extremes 

 are, in fact, represented in the behavior of Euglena, which will 

 be considered in another paper. Whether they also represent 

 two different mechanisms of orientation is a question for the 

 future to decide. 6 That they do not involve the selection of 

 random movements there appears to be no doubt. 



In the following account of the reactions of Porcellio scaber, 

 it will be seen that although random movements are common 

 they can readily be distinguished from the forced movements 

 that occur in definite predictable directions in response to dif- 

 ferential stimulation of symmetrically situated photoreceptors. 

 But such phototropic movements not only exist; they are large 

 factors in the orientation of Porcellio to light. This is true also 

 for Allolobophora sp. and the larvae of an undetermined blow fly. 



Porcellio scaber, a species of sow-bug, or wood louse, very 

 common on the Pacific coast, is a typical symmetrical isopod 

 with a pair of compound eyes set far apart in the head segment, 

 and two pairs of antennae, of which the second antennae are 

 conspicuous tactile organs, restlessly active during locomotion. 

 The subequal walking appendages and the body in general are 

 also sensitive to contact stimuli. Of other sense organs it is 

 unnecessary now to speak. 



• Since this was written, a paper by Dr. F. W. Bancroft, in the Journal for Ex- 

 perimental Zoology for November, 1913, appears definitely to have settled the 

 question, for Euglena at least, in the affirmative. 



