116 HARRY BEAL TORRE Y AND GRACE P. HAYS 



uli, effectively masked the heliotropic movements so apparent 

 under other conditions. Similar pronounced movements were 

 frequently seen when a larva, crawling out over the edge of 

 the glass plate on which it was being observed, would free the 

 anterior third or half of its body. It would then wave this free 

 portion about much in the manner of a leech. Dryness of the 

 substratum may produce similar effects. Such behavior suggests 

 the probability that even the small random or trial movements 

 of the anterior end that ordinarily accompany locomotion are 

 controlled — their amplitude, perhaps being determined — to some 

 extent by contact stimuli. 



7 



It is possible then, to distinguish between random movements 

 that have no connection with photic stimulation, and movements 

 that Mast calls trials, but are conditioned by photic stimula- 

 tion. For convenience in further analysis, it will be desirable 

 to distinguish between two groups of reactions thus conditioned. 

 In the one may be placed reactions to high intensities of light, 

 such as direct sunlight; in the other, reactions to lower inten- 

 sities. All of these reactions are regarded by Mast as trial 

 movements similar to the avoiding or shock reactions of the lower 

 organisms. The reactions of the second group — however we 

 may view them as " trials " — do indeed resemble those reac- 

 tions of such a form as Euglena, that are in the same general 

 direction with reference to the source of light. The reactions 

 of the first group, however, occur either toward or away from 

 the source of light. They are non-directive with reference to 

 the source of light. 



This distinction is emphasized by our observations on earth- 

 worms and fly larvae. When light was allowed to fall from the 

 side upon the extended anterior end of either of these forms, 

 the first movement of the anterior end was for certain intensi- 

 ties of light away from the latter, whether directed toward or 

 away from the light, when exposed. 



8 



To eliminate as far as possible all non-directive reactions 

 from the behavior of Porcellio to light, in order to discover any 

 directive, tropic movements of orientation that might be present, 

 we adopted two very simple methods. The first consisted in 



