460 A. M. Banta 



is 1.79; the same average for Caecidotea is ^.t,J. Asellus, out of 

 120 trials, 20 upon each of six animals gave but one reaction to 

 stimulation by grade 7 (169 units of momentum) and no reactions 

 were gotten from stimulation below this grade. Caecidotea con- 

 tinued to respond to the lower grades and to some extent even to* 

 the lowest grade used (54 units of momentum). The close agree- 

 ment of results obtained hy grades 5 and 6, in which different 

 bodies strike with the same momentum removes any reasonable 

 doubt as to the justification of considering the effectiveness of the 

 stimulation to be dependent upon the mornentum. 



As already noted it sometimes occurs that animals upon which 

 tests were being made were moving at the time ot the test. Un- 

 der such circumstances the movements of the animals were often 

 arrested. An arrest of movements due to a stimulus is of course as 

 definite a reaction as the inciting to movement by a stimulus. 

 Such arrests of movements were recorded with the other reactions. 



One of the most delicate reactions was a twitching of the flagel- 

 lum of the antenna or a stroking of the antenna by the gnathopod. 

 This reaction suggested the possibility that the flagellum is par- 

 ticularly sensitive to such mechanical stimulation. 



The results of the foregoing experiments may be summarized 

 as follows: 



1. Falling bodies having the same momenta produce virtually 

 the same reactions. These become more numerous as well as 

 more vigorous with increase of momentum. 



2. Asellus responds much less often than Caecidotea to such 

 stimuli of whatever grade. 



3. Asellus responds less vigorously than Caecidotea to such 

 stimuli; the average of the vigor of responses for the whole num- 

 ber of responses for Asellus was 1.79 and for Caecidotea ^-TfJ. 



4. The threshold of stimulation for Asellus was found to be 

 near that of grade 5 and 6 (219 units of momentum), while the 

 threshold for Caecidotea was approached only with the lowest 

 stimulation used, grade 11 (54 units of momentum), a grade only 

 \ as high. 



