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* Experiments unpublished. 
