262 University of California Publications in Zoology ["Vol.11 



thirds of the body, often a dozen or more of them at the same 

 time will be found exercising the respiratory movements. In 

 the following experiments, an aquarium eight by nine by eight 

 inches, containing about five inches of water, was allowed to 

 remain for a couple of hours undisturbed on a one-inch, well- 

 seasoned pine board sixteen inches in width and approximately 

 seven feet in length. This board was rested on two laboratory 

 tables six feet apart, and the flexibility provided the proper con- 

 ditions for the application of a stimulus of the desired intensity. 



The undulatory respiratory movements were in progress 

 among the leeches. These movements are much like the swim- 

 ming movements, except not so vigorous, the posterior sucker 

 remaining atatched. During the execution of them the body 

 of the leech may sway about in various directions, and often 

 this fact proves a cause of interference in the progress of the 

 experiment, owing to the contact of the body with that of another 

 individual. The .jar was made by tapping the board verj^ slightly 

 with tlie closed fist, allowing this to drop gently through a dis- 

 tance of about one foot above. By this means an approximately 

 uniform stimulus could be secured. 



It is somewhat unusual to secure a specimen continuing its 

 rhythmical movements for several hours continuoiLsly, but this 

 was observed in some cases. The influence of slight jars on 

 these movements in a single leech is indicated in an accompanying 

 table (see table IV). A slight jar of the dish was made at inter- 

 vals of thirty seconds for twenty successive times, the duration 

 of the cessation of response being measured by means of a stop 

 watch. Several series of these stimuli were made with varying 

 intervals between the series. Even so slight an interval as two 

 minutes, as is shown in the last column of the table, serves to 

 allow the animal partially to recuperate its original responsive- 

 ness. 



In order to determine the range of individual variation in 

 this acclimatization response, several other individuals were tasted 

 out at intervals of thirty .seconds. Considerable variation occurs 

 among them in regard to duration of the first cessation period 

 and succeeding ones, some individuals acclimatizing much more 

 readily than others. After several negative responses, the stim- 



