264 University of Calif ornia Publications in Zoology [Vol. 11 



them and the window which affords their greatest source of light, 

 an immediate contraction is the usual response, the animal hug- 

 ging the bottom of the dish. Allow the board to remain inter- 

 posed until the leech has again resumed its movements and then 

 remove the obstruction. No response to this increased intensity 

 of light is evoked, the undulatory movements continuing. This 

 is the same condition found by Loeb (1893) and Hargitt (1906) 

 to hold in certain of the tubicolous annelids, where only a de- 

 crease in intensity seems to act as a stimulus to the animal. 

 However, in the case of Di^ia any considerable increase in the 

 intensity of light is sufficient to cause cessation of the undulatory 

 movements, and eventually movement to a less lighted portion 

 of the dish. A group of leeches performing these undulatory 

 movements was illuminated with two 25-watt Mazda lights. The 

 movements were at first rather vigorously accelerated, but later 

 the animals completely withdrew underneath the stone or moved 

 to a less lighted portion of the dish. 



Acclimatization to Jars as Expressed in Number of Seconds Cessation 

 OF Undulatory Movements in Five Specimens op Dina microstoma 



