42 Herbert Eugene Walter 



due to what Graber ('83, p. 229) has called a 'dermatropic' 

 function." 



Bardeen ('01a, p. 13), speaking of Planaria maculata, states that 

 "susceptibility to light is apt to become lost if worms are kept in 

 captivity," and he notes the fact, already brought out byChichkoff 

 ('92), that pigment becomes reduced in sunlight. Hesse had 

 previously emphasized the point that the pigment of the eye of any 

 organism has in itself primarily nothing whatever to do with light 

 perception. Bardeen further found that small pieces of planarians 

 capable of locomotion will respond to light in the same way as 

 uninjured animals, and he notes ('01a, p. 13), that the worms 

 seem "to move about more by night than by day." In a later 

 paper ('01b) he speaks of the fact that when a dish containing 

 planarians is brought into light the worms are commonly roused 

 to activity, although how far such activity is due to light and how 

 far to mechanical disturbances he does not make clear. 



Lillie ('01), experimenting upon the regeneration of Dendro- 

 ccelum lacteum, discovered that posterior headless parts fail to 

 give the typical reaction to light and are incapable of regeneration. 

 He draws the conclusion ('01, p. 132) that "any symmetrical piece 

 of Dendroccelum capable of regeneration tends to come to rest in 

 the shaded part of the dish precisely like a normal individual" and 

 that parts incapable of regenerating "also become incapable, 

 after a day or two, of performing the usual reactions to light." 

 These results on Dendroccelum, it will be seen, are similar to those 

 Loeb obtained in experiments upon the polyclad Thysanozoon. 



Curtis ('02) reports from laboratory observations 42 cases of 

 fission in Planaria maculata, of which number 39 occurred between 

 10 p. m. and 6 a. m. He adds, however ('02, p. 524), that "this 

 did not seem due to the amount of light to which the animals 

 were subjected during the day, for some of the dishes were so 

 shaded that there was practically no light, day or night, except 

 when they were being examined, and the division was the same 

 in these as in others which were exposed to full daylight. " A case 

 of division in Bipalium also occurring by night is described by 

 Lehnert ('91). 



In a contribution to the geographical distribution of Planaria 



