364 PROCEEDINGS OF THE AMERICAN ACADEMY. 



this arrangement is greatly increased by the lateral extension of the 



Figure 8. riiotoiuieroyrapli of a portion of the margin of tlie head of Bipa- 

 liuin, showing the position antl arrangement of the eyes. X 70. 



head, which allows room for a greatly increased number of eyes in its 

 anterior border. Taken as a whole, this arrangement might be roughly 



compared to a single convex mo- 

 saic eye, such as is found, for 

 example, in the Entomostraca. 



Below are described the re- 

 sults of physiological tests with 

 lights of different sizes, in an 

 attempt to determine in how far 

 the eyes may act in this way. 



It is a well-known fact that 

 most planarians avoid the light 

 (the exceptions being certain 

 chlorophyll-bearing forms), and 

 they have long been used for 

 work in phototropisra. Bipa- 

 lium appears never to have been 

 used for experimental purposes, 

 although the fact that the land 

 planarians share with the water 

 inha1)iting forms aversion to 

 light was remarked upon more 

 than sixty years ago by Darwin ( '44). That they are also nocturnal 

 in their habits has been commented upon by various authors since that 



riGDRE 0. Diagram of tlie head of Bl- 

 paliitin, to sliow how light coming from 

 different directions (indicated by the ar- 

 rows) may alfect only certain of the eyes. 



