The Reactions of Planarians to Light 123 



The moment, however, that the eyes were in shadow the worm 

 would elongate and frequently begin strikingly active movements. 



It has already been shown (Table XIV, p. 77) that all the 

 different species of planarians upon which experiments were made, 

 traveled at a faster rate when they were started with the ante- 

 rior end pointed toward directive light than when away from it. 

 A reason may be offered for this characteristic increase in rate on 

 the ground that the anterior end was plainly subjected to stronger 

 stimulation when directed toward the light than when pointed 

 away from the source of the stimulus. In the latter instance it 

 was not only turned away from the source of the stimulus but 

 was shielded also from the light to a considerable extent by the 

 shadow formed by the posterior part of its own body. 



Again, when a small beam of sunlight passing through a pin- 

 hole in an opaque screen was directed locally to different parts of 

 a gliding Planaria maculata, it was found that tropic response 

 would occur in case one side of the anterior end was illuminated, 

 and that it was not necessary for the eye itself to be included in 

 the illuminated area to obtain such responses. However, when 

 the middle of the body or the posterior end was similarly stimu- 

 lated the worm could not be made to turn. 



From the foregoing observations it seems probable that the 

 photoreceptive apparatus of planarians is mainly but not exclu- 

 sively located in the anterior end of the body and that considerable 

 specific or generic difference may exist with respect to the extent 

 of the distribution of additional light-receiving organs over other 

 parts of the body. It is interesting to note in passing that Gamble 

 and Keeble ('03) found that in the case of the green rhabdocoele 

 Convoluta roscoffensis the sensitiveness to light was at the ante- 

 rior end of the body only. 



Concerning the relative sensitiveness to light of the dorsal and 

 ventral surfaces of planarians, a set of experiments was performed 

 on Planaria gonocephala in which the results show an absence 

 of any marked differentiation in this regard. It is well known 

 that in the matter of response to a thigmotactic stimulus the dor- 

 sal and ventral surfaces of a planarian show a very striking differ- 

 ence. Indeed, the dorsal surface is negatively thigmotactic to 



