462 A. M. Baiita 



the stimulus was applied and in such cases often an arrest of move- 

 ment occurred. This was indicated by an "A." Table VII 

 shows the detailed records of the hfteen pairs of individuals 

 tested in this manner. 



An examination of Table VII discloses the fact that in every 

 case where a pair of the two species were subjected to test, the 

 Caecidotea was the more responsive of the two. Asellus made 

 on an average for the 15 individuals tested 6.8 responses in 20 

 trials, with an average vigor of 1.39, while the 15 Qnecidotea aver- 

 aged II. 8 responses in the same number of trials, with an average 

 vigor of 3.02. Hence the number of responses made by Caeci- 

 dotes was nearly twice as large as the number made by Asellus 

 and at the same time they were on the average considerably more 

 than twice as vigorous. 



Very often, particularly with Asellus, the animal responded by 

 trembling or quivering movements of the flagellum of the antenna 

 or a quick stroking of the antenna with the gnathopod. Such 

 reactions occurred often with this sort of stimulation as well as 

 with the stimulation from the concussions of falling balls, but to 

 stimulation by bristles or locally applied currents of water it was 

 scarcely ever noted. These results suggest that the response to 

 mechanical vibrations may be more or less localized in the flagel- 

 lum of the antenna. 



Ihe following statements summarize these results: 



1. Asellus is less sensitive to such vibrations than Caecidotea. 



2. Asellus responds to such stimulation scarcely more than 

 half as often as Caecidotea. 



3. Asellus responds to such stimulation less than half as 

 vigorously as Caecidotea. 



4. Arrest of movements is often caused by this form of stimu- 

 lation. 



5. The fact that the antenn;e are so often whipped about and 

 rubbed by the gnathopod during such stimulation suggests that 

 they are more sensitive to this stimulus than other parts of the 

 body although to pure tactile stimulation tney are scarcely at all 

 sensitive. 



