224 University of California Publications in Zoology [Vol. 11 



Random movements play a considerable role in the responses 

 of the leech to light. Tests made on Glossiphonia indicate that 

 the higher the degree of illumination the greater the initial excess 

 of random movements. This fact is apparent upon comparing 

 the accompanying diagrams (figs. 3, 4). These were secured by 

 placing upon a rigid glass plate a flat glass dish containing the 

 leech to be tested. The leech was placed in each case directly 

 oriented toward the source of light, and this was then turned 

 on. With a sharply pointed wax pencil the course of the leech 

 was traced on the glass underneath the leech itself as it moved 

 forward. From this glass plate a copy of the route pursued by 

 the leech was taken. Out of many surplus movements, the leech 

 follows up only those that tend to rid it of the stimulating agent. 

 Orientation of the body of the leech so that its two sides are 

 equally stimulated by the light occurs, however, in only a rela- 

 tively few cases. 



Fig. 3. — Influence of 100-watt Mazda light on the direction of move- 

 ment of five leeches of the species Glossiphonia stagnalis ; (e) representing 

 the responses of an individual which left the bottom of the dish for the 

 surface film. 



