40 Herbert Eugene JValter 



light relations of planarians in various papers by Loeb, whose 

 important contribution in 1890, "Der Heliotropismus der Thiere 

 und seine Uebereinstimmung mit dem Heliotropismus der Pflan- 

 zen," paved the way in general for all work on this subject. He 

 found ('93b, p. loi) that Planaria torva is not "heliotropic" in 

 the strict sense, but rather "unterschiedsempfindlich," that is to 

 say, it did not always move aw^ay from the source of the light in the 

 direction of the rays and remain as far removed as possible, but 

 moved about more or less at random, coming to rest in some area 

 of lessened light intensity. 



In a later paper Loeb ('94, p. 255) states that when planarians 

 are suddenly brought into the light they begin to move, an increase 

 in light intensity leading to activity and conversely a decrease, to 

 rest. The grounds for this conclusion are not made clear. He 

 further confirms the view that planarians are active at night, com- 

 ing to rest in situations of lowered light intensity in daytime. 



In further experiments P. torva when decapitated was found 

 to react to light precisely as normal worms do with the difference 

 that the reaction required more time. Thysanozoon brochii, a 

 polyclad, on the other hand, lost its power to respond to light when 

 the eyes and brain were amputated, from which Loeb draws the 

 conclusion that animals which are closely related morphologically 

 may exhibit wide physiological differences. 



Hesse ('97), in his classic study of the anatomy of the turbel- 

 larian eye, mentions some experiments and observations on the 

 behavior of planarians and in addition makes valuable contribu- 

 tions to the morphological basis of light reactions. He observed 

 that planarians become active at twilight and he also experimented 

 upon decapitated worms, apparently without being aware of the 

 previous work by Loeb, with whose results his own agreed. He 

 found, as Loeb did, that worms deprived of eyes finally come to 

 rest in areas of lesser intensity much as do normal worms but after 

 a longer time. Hesse found, too, that Dendroccelum lacteum 

 came to rest in the dark 119 times out of a possible 120, whereas 

 with Euplanaria (Planaria) gonocephala the same result was 

 effected in only 88 out of 120 times, notwithstanding the fact that 

 the latter has more highly developed eyes than Dendroccelum. 



