REGULATION AND FISSION IN PLANARIA. 123 



true of the Californian species. My observations have led me to 

 believe, however, that the level varies to a considerable extent. 

 The direct cause of separation seems to me to be a difference in 

 reaction of the two parts ; a certain portion of the postpharyngeal 

 region attaches itself firmly while the other parts of the body 

 attempt to move forward. Since the motor power for the for- 

 ward movement, /. c., the cilia and the margins of the body --is 

 applied not at the anterior end only but throughout the whole 

 length of the part anterior to the attached region the greatest 

 strain will occur just anterior to the attached region and rupture 

 will occur here even in the absence of any preformed zone of 

 weakness. Personally I am inclined to doubt the existence of 

 any such zone of weakness, but I think the facts show very 

 clearly that under ordinary conditions Planaria consists physio- 

 logically of two zooids. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 April, 1906. 



