240 University of California Publications in Zoology [Vol. 11 



2. Diurnal. Rhythmical Behavior 



The leeches experimented with in this work, as has been 

 stated, are under normal conditions negatively phototaetic. This 

 is well evidenced by the fact that they persistently remain under- 

 neath stones or among the debris of their environment during 

 the day. Upon coming into the laboratory at night I observed 

 many times that the leeches of both of the genera Dina and 

 Glossiphonia were crawling actively about in the aquaria. In 

 many instances the glossiphonids were observed by the hundreds 

 crawling about on the sides of the dish, exercising imdulatory 

 movements, or sometimes moving along underneath the surface 

 film much in the same manner as does the fresh-water planarian. 

 Dina was usually to be found crawling about on the bottom of 

 the dish. 



The diurnal rhythm appeared so pronounced in Glossiphonia 

 stagnalis that it was thought worth while to test its possible per- 

 sistence under uniform and constant conditions of illumination. 

 When a jar containing several dozens of adults was placed under 

 constant light in a dark room it was found that the leeches re- 

 mained underneath the stones in the dish for as long as a week, 

 the length of the experiment. Observations were made daily 

 at intervals of a few hours from eight o'clock in the morning 

 to a varying period of nine to eleven in the evening, and it was 

 very rarely that a single specimen was found active in the dish. 



The jars of specimens kept under constant darkness showed, 

 within a half hour after being placed in the dark room, a very 

 noticeable activity and movement of many of the leeches to the 

 sides of the dish. This response was found to continue for several 

 days, not any of the leeches, however, continuing active during 

 the whole of this period. 



These results seem to indicate that the diurnal rhythm in 

 these forms is very largely the result of the direct action of light. 

 However, certain other factors must play an accessory role in 

 the production of periods of rest and activity. Thus fatigue 

 as the result of continued activity would cause the animal to 

 come to rest in conditions of either darkness or light. Satiety 

 would have much the same effect, as is shown in experiments 



