122 Herbert Eugene Walter 



o 



are more deeply buried from the light than when they are in the 

 expanded position assumed while gliding. 



b Photoreceptors 



What is the photoreceptive apparatus of the planarian ? Is it 

 made up of the eyes only, or partly of nerve-endings or of some 

 special morphological elements homologous perhaps to the pho- 

 toreceptor cells in the integument of the earthworm as described 

 by Hesse ('96). Or does the central nervous system, the ciliary 

 apparatus, or the musculature receive the stimulus directly with- 

 out the mediation of special sense organs ? 



Although these questions were not made the subject of parti- 

 cular investigation in the present study of planarian behavior, 

 certain facts incidentally appear from the observations made for 

 other purposes which bear directly upon these inquiries and may 

 serve as a basis for a brief discussion of the nature and location 

 of the photoreceptive apparatus of planarians. The presence of 

 eyes in the anterior part of the body, together with the wigwag 

 movements which often take place in the same region when a 

 variation occurs in the light conditions, point directly to the con- 

 clusion that the anterior end of the worm is more responsive to 

 light than the posterior end. The fact that many planarians con- 

 tinue to react to light with considerable definiteness after the 

 whole anterior end of the body is removed, indicates that this 

 region does not necessarily contain the entire photoreceptive 

 apparatus. Decapitated individuals of the species Dendro- 

 ccelum lacteum, it should be noted, seem to be exceptional in this 

 respect since, according to Lillie, they fail to react to light. 



In further support of the supposition that the anterior end of 

 the planarian is the portion most sensitive to light it was found 

 that the skioptic response of Bdelloura Candida is confined not 

 only to the anterior end but particularly to the region directly 

 including the eyes. Observations repeatedly showed that if 

 Bdelloura was allowed to come to rest in a field illuminated from 

 above only, a sharp narrow shadow thrown across its body pro- 

 duced no visible response unless the shadow included the eyes. 



