The Reactions of Planarians to Light 45 



1 Planarians are nocturnal, seeking the dark when exposed to 

 light. 



2 The eyes are useless in finding food. 



3 The anterior end of the body is the part most responsive 

 to light 



4 Decapitated worms act normally except for a slower reac- 

 tion tmie. 



5 Orientation to light depends largely upon the character of 

 the pigment cups of the eyes. 



6 The relative energy of the response is dependent upon the 

 intensity of the light. 



7 Pigment is reduced in sunlight. 



8 Pieces of worms which are large enough to move or regener- 

 ate react to light. 



9 Fission may occur more readily in the dark. 



10 Different species respond differently to light. 



11 Light reactions diminish during "captivity." 



12 Planarians are "unterschiedsempfindlich" instead of "helio- 

 tropic. " 



III Material 



The species principally used in the following investigations 

 were Planaria maculata Leidy; Planaria gonocephala Duges; 

 Phagocata gracilis Leidy; Dendrocoelum lacteum Oersted; and 

 Bdelloura Candida Giard, all of which are inhabitants of fresh 

 water except Bdelloura, a salt-water species, found living semi- 

 parasitically on the horseshoe crab (Limulus polyphemus). Some 

 observations also were made upon a cave planarian, that as yet has 

 not been identified but which may belong to the genus Phagocata. 

 This interesting worm w^as kindly placed at my disposal by Dr. 

 A. M. Banta. 



At any season of the year an ample supply of fresh material was 

 easily obtained except in midwinter, when it was necessary to cut 

 through the ice and dredge up from the bottom water-weeds to 

 which the w^orms cling. 



The source of supply for Planaria gonocephala was a small 

 pond to the west of Fresh Pond in Cambridge, Mass., while Pla- 



