The Reactions of Planarians to Light 39 



doubts whether the eyes are of service in finding food and says 

 of worms under aquarium conditions ('14, p. 107), "If remaining 

 a considerable time unchanged, the planariae decrease more rapidly, 

 they become languid, scarcely moving either by the influence of the 

 light or heat, and at last adhere entirely to the side of the contain- 

 ing vessel, where they perish. " 



Duges ('28) observed that when light is concentrated by means 

 of a lens upon either Dendrocoelum ( ?) or Planaria, movement 

 results which is most pronounced when directed toward the ante- 

 rior end of the worm. He tested the effects of direct sunlight and 

 of diffuse daylight as well as of candle-light, and concluded that 

 the response increases with the intensity of the light. The non- 

 dioptric character of the eyes he has described remarkably well 

 for one working so long before the days of the microtome, and his 

 conclusion, already suggested by Dalyell and later confirmed by 

 Kennel ('88), and others, that the eyes play no part in the finding 

 of food, is noteworthy. He also notes that planarians seek the 

 dark. 



Dalyell ('53, p. 99), in a later volume says, "On April 29 I pro- 

 cured a fine specimen of Planaria cornuta, which spawned soon 

 afterward. The spawn had been breaking up for two or three 

 days preceding May 24, when multitudes of extremely minute 

 yellow specks were seen swimming in the water. Their motion 

 was sufficiently active, without being very quick; it was pursued 

 in all directions and the spawn being contained in a small cylin- 

 drical jar, the specks crowded to the sides next the light whereon 

 numbers remained almost stationary." Again ('53, p. 104), 

 "When withdrawn from the dark the young Planariae rose in great 

 numbers toward the surface of the water, congregating on the 

 sides next the light. " It is extremely doubtful whether the organ- 

 isms here described were really young planarians. It is more 

 likely that they were the young of some other aquatic animal. 

 Dalyell correctly describes Planaria lactea (Dendrocoelum lac- 

 teum T) as being nocturnal. He observed that numbers of this 

 species, beginning activity in the evening, rose on the sides of the 

 jar, although many had descended again by morning. 



More recently attention has been specifically directed to the 



