BYDROTROPISMS OF FRESH-WATER INVERTEBRATES. 



39 



is repeated over and over again, using one sheet of paper for 

 each five experiments, until the creature has worked with its 

 head fari 11:^ all eight of the radii tips. This gives forty experi- 

 ment- and i- called a series. Work is continued until one has 

 secured record- of as many series as is thought necessary. In the 

 work recorded in this paper an attempt was made to secure 

 between twelve and thirteen series with each species investigated. 

 In practice it is not always possible to secure a complete series 

 n an individual. Sometimes the individual would escape 

 before the series was completed, at others fatigue would put an 

 end to the experiments at an early stage. In some cases it was 

 necessary to return the specimen to the water at the close of 

 each experiment, instead of at the close of each five. Two invecti- 

 iMtor- (.in do more effective work than one; one giving his en- 

 ''' time to looking after the animal and the other recording what 

 happen^. This is especially important when dealing with forms 

 that fly and with crawling forms that move rapidly. After a 

 little experience one recognizes the movements preliminary to 

 flight. I'sually a small glass cover placed over the specimen for a 

 moment will check the attempt to fly. 



In the work performed in connection with this article, on 

 returning from the field the results of the days work were re- 

 corded on tables with the following captions; 



TAHU-: I. 



Later these tables were condensed into those published in this 

 article. 



In the experiments recorded in thi- article the side of the 



