44 Herbert Eugene Walter 



Moseley ('74, p. iii) states that "land planarians are probably 

 all of them nocturnal in habit." Speaking of the Ceylon land 

 planarians in particular he says: "They are found in dark places, 

 such as under large fallen leaves, and in confinement they coil 

 themselves up away from light. " He mentions also the fact that 

 Planaria torva and Dendrocoelum lacteum choose the dark side 

 of the vessel in which they are contained. 



As has already been mentioned, Lehnert ('91) found Bipalium 

 kewense undergoing fission in the dark. Both Bipalium and 

 Geodesmus, he says, seek contmually to hide in shadowy places 

 avoiding even diffuse daylight. Concerning the degree of light 

 perception possessed by planarians, he offers the opinion ('91, p. 

 326) that "Bipalium scheint mit seinem Augen die Umrisse von 

 Gegenstanden in Lichte wahrnehmen zu konnen." 



Hogg ('97) notes that Bipalium is nocturnal in habit, remaining 

 sluggish during the day. 



Only incidental references to the polyclads are found bearing 

 upon the question of light reactions, as for example this sentence, 

 which occurs m Lang's exhaustive monograph ('84, p. 641), "Die 

 meisten Arten scheuen das directe Sonnenlicht. " The behavior 

 of Thysanozoon with reference to light has already been mentioned 

 (Loeb, '94). 



Concerning the light reactions of the rhabdocoeles, especially cer- 

 tain green forms in which the green cells are probably symbiotic, 

 a considerable literature may be found. The principal papers 

 relating to these forms are as follows: On Convoluta schultzii, 

 by Geddes ('79), Barthelemy ('84) and Delage ('86); on Convo- 

 luta roscoffensis, by Haberlandt ('91), Bohn ('03a, '03b, '03c), 

 Gamble and Keeble ('03) and Fiihner ('06). Vortex viridis and 

 Mesostomum viridatum ( .?) are discussed by Schultze ('51), 

 von Graff ('84) and Sekera ('03). A resume of these papers is, 

 however, out of place here, since the presence of green cells in the 

 organisms involves an entirely different problem from that 

 which is under consideration. 



The foregoing historical sketch furnishes the basis of the follow- 

 ing general summary of facts which have thus been established with 

 more or less certainty regarding the reactions of planarians to light. 



