226 TJniversity of California Publications in Zoology [Vol. 11 



well defined. In these experiments the entire body was illumi- 

 nated on the side toward the light. Under such conditions, as 

 can be noted from the accompanying table, there is a decided 

 excess of turns away from the source of light. Continued illumi- 

 nation produces, in the cases where the first response is indif- 

 ferent, usually a turn away from the source of light. 



The accompanying diagrams (figs. 3, 4) represent the paths 

 pursued by several Glossiphonia in their movement away from 

 the light. A study of these will indicate the part played by 

 direct turns in their orientation. Although the initial excess 

 of random movements under the influence of a 100-watt Mazda 

 light is greater than that caused by a 25-watt light, there is a 

 decided diiference in the orientation effected. As the leech moves 

 farther away from the 25-watt light the number of random 

 movements becomes much increased. When orientation is effected 

 as the result of the 100-watt light it continues with little random 

 movement across the remaining length of the 25 em. dish. 



Fig. 4. — Influence of 25-watt Mazda light on the direction of move- 

 ment in four leeches of the species Glossiphonia stagnalis ; h and 6' repre- 

 senting the aberrant behavior of a single individual. 



