The Reactions of Planariaus to Light 7 1 



The difficulties presented by paper as a reflecting surface largely 

 disappeared by the substitution of a plain mirror in its place, since 

 the surface of a mirror is such that all the light striking it at right 

 angles is reflected at right angles. When, therefore, an unmounted 

 mirror was brought into contact with half of the under surface of 

 the aquarium floor the whole field was thereby divided into two 

 regions sharply separated from each other. Of these one was 

 supplied with light from above only, while the other received the 

 same light plus nearly an equal amount reflected from the mirror 

 below. With the aid of this device an increase of 11 per cent was 

 gained over the responses obtained when white paper instead of 

 a mirror was used as a reflector. Both Phagocata and Dendro- 

 coelum were tried by this method. In 76 per cent of the trials 

 made, i. e., in 125 cases out of 165, the worms showed a visible 

 modification in their behavior on reaching the boundary of the 

 two areas of light. It was nevertheless decided that this method 

 was an uncertain test of behavior, since the body of the worm, 

 although fairly translucent, would by no means allow all the light 

 that fell upon it to pass through and be reflected, and consequently 

 the diff^erence of the two mtensities to which it was being sub- 

 jected could not be easily estimated. 



Summary. When sudden light or dark envelops planarians 

 (Dendrocoelum, Phagocata and Planaria) the response, if any 

 occurs, is often not immediate. 



No one of the intensities of light which were employed in these 

 experiments when introduced suddenly was sufficient to make the 

 worms forsake gliding for crawling. 



Sudden dark calls out more responses than sudden light, while 

 the number of responses increases with an increasing interval of 

 exposure to the stimulus. Bdelloura is decidedly "skioptic. " 



Worms encountering the edge of a reflecting area which increases 

 the intensity of the light without introducing any other barrier, 

 show a marked degree of response. The percentage of response 

 is considerably larger when a mirror instead of white paper is used 

 to produce the reflecting surface. If worms are allowed to pass 

 from one intensity to another sharply separated from it, their 

 responses are more frequent upon passing into the lesser intensity 



