The Reactions of Planarians to Light 



111 



The lessened accuracy in orientation among the smaller worms, 

 as compared with the larger ones, helps to support the hypothesis 

 that reduction in size entails proportionate reduction in the 

 photoreceptive apparatus. The fact that Dendrocoelum lacteum 

 forms an apparent exception to this general rule may also be 

 taken a? evidence that in this case the photoreceptive apparatus 

 is more localized than in the other worms studied and conse- 



TABLE XXXII 



The average deviation (expressed in degrees^ at the circumference of a circle 10 cm. in diameter, of 

 large and small worms, each lot consisting of 5 individuals. Each worm was headed successively 

 toward, away from and at right angles in both directions to incident light. The actual sizes of 

 the worms were the same as in Table XXXI 



quently does not suffer a proportionate decrease when the size of 

 the body becomes less. It is furthermore quite possible that a 

 sexually mature planarian may on that account behave differently 

 in light than an immature one. For instance, Yerkes ('03) states 

 that in the case of the hydromedusa Gonionemus murbachii, the 

 sexually mature individuals are the ones most sensitive to light, 

 and Schouteden ('02) found the young of Daphnia positive, while 

 the adults were negative to light. 



Finally, Harper ('05) has shown that in the case of the earth- 

 worm the degree of sensitivity to light depends upon the degree 

 of contraction or expansion of the body, since the photoreceptor 

 cells — which in the earthworm He interstitially at the bases of the 

 epithelial cells — are more exposed to stimulation when the worm 

 is expanded and conversely more shielded when it is contracted. 

 It is more than likely that planarians offer a parallel instance 

 and that their comparative indifference to light stimulation when 

 in the relaxed resting position is due to the fact that then they pre- 

 sent a more rounded contour and consequently their photoreceptors 



