50 THE TROPISMS 



clops. In the latter a weakly alkaline condition seems to 

 induce a negative reaction. The experiments of Mr. 

 Jackson on Hyalella, a fresh water amphipod which is 

 ordinarily negative in its response, have shown that a variety 

 of substances cause a positive reaction. Mrs. Michener* in 

 experiments on a number of diverse forms finds that if the 

 response is negative it may be made positive by various 

 chemicals, although normally positive forms are rarely if 

 ever rendered negative. A wide range of chemicals was 

 experimented with and a positive response was evoked by 

 acids, salts, alkalies and various other substances, as in the 

 experiments of Mr. Jackson, showing that there is no definite 

 relation between the nature of the response and the class of 

 chemical compounds employed as stimuli. 



The effect of contact stimuli on phototaxis affords one of 

 the most curious phenomena connected with the modi- 

 fication of the responses of organisms to light. In many 

 cases contact causes an immediate reversal of the response. 

 Miss Towle found that negative specimens of the ostracod 

 Cypridopsis were rendered positive by being picked up in a 

 pipette and dropped out again. Negative specimens when 

 colliding with one end of a dish straightway became positive 

 and swam to the other end. Yerkes obtained similar re- 

 sults in another ostracod, Cypris. Daphnia, which is ordi- 

 narily positive, may be made negative for a very short time 

 by repeatedly picking it up in a pipette. The copepod Temora 

 longicornis, which is usually negative, was found to become 

 temporarily positive by being shaken (Pouchet). Positive 

 specimens of Labidocera, on the other hand, may be made 

 temporarily negative by handling them in a pipette (Parker). 



Possibly allied to these effects is the curious reversal of 

 phototaxis which occurs in certain terrestrial amphipods when 

 thrown into water. Having observed that the terrestrial 



* Experiments unpublished. 



