38 Herbert Eugene Walter 



II Historical 



Our knowledge of planarians, as of most other animals, has 

 passed through certain historical phases, during which emphasis 

 has been laid first upon taxonomy and anatomy and latterly upon 

 embryology and zoogeography. The results of these various 

 forms of investigation are highly important since they make the 

 foundation for all future work upon this group of animals. They 

 have, however, only an indirect interest in the present connection 

 and do not, therefore, require review. 



Perhaps the most modern advance in our knowledge of pla- 

 narians is represented by the school which treats of them as living 

 objects whose individual behavior is to be intimately correlated 

 both with their structure and environment. The most noteworthy 

 contribution from this standpoint has been made by Pearl ('03), 

 who has analyzed in considerable detail the reactions of fresh- 

 water planarians (notably Planaria maculata, Planaria doroto- 

 cephala and Dendrocoelum lacteum) to various stimuli. He has 

 not, however, discussed the effects of light except incidentally. 



The earliest reference to the relation of planarians to light is 

 by Dalyell ('14). In his interesting volume on planarians a great 

 number of keen observations upon the general habits and struc- 

 ture of planarians are made, which have since been confirmed, 

 together with certam statements which have not fared as well with 

 the advance of scientific knowledge. 



He makes the statement ('14, p. 9) that "most planariae court 

 the light indeed;^ but P. flexilis rather inclines to shun it, less, we 

 may conjecture, from being warned of its presence by the specks 

 or eyes, than from some disagreeable sensation produced on the 

 body." Again, referring to P. felina ('14, p. 46), "This planaria, 

 like the rest of its genus, is powerfully excited to motion by the 

 presence of light. If a number be confined in a glass vessel, the 

 whole assemble m a quiescent state, on the side next the light.^ It 

 is a little surprising that Dalyell should have received the impres- 

 sion that the majority of planarians "court the light," since he 

 clearly points out the nocturnal habits of these worms. He 



' The italics are mine. 



