THE SENSORY RESPONSES OF CHITON 241 



LiCl: Much as with NH4CI, but reactions noticeably weaker; re- 

 sponses not initiated so quickly, nor so completely carried out. 



NaCl: Responses in the mouth region extremely weak; on the edge 

 and the sole of the foot, still weaker; the gill responses possibly as 

 decided as with LiCl. The edge of the foot moves at first toward the 

 stimulating hquid, then away. 



CaCk: No reactions from the gills; on the mouth region, and the 

 edge and sole of the foot, stronger responses than with NaCl. 



MgCk: The gills were raised toward the source of application; weak 

 negative response from the sole of the foot; the edge of the foot at 

 first pushed locally toward the fluid — as with NaCl — then away if the 

 application were prolonged. In the mouth region, fairly strong 

 contractions. 



From these findings it appears that the cation order of decreas- 

 ing stimulating power is 



K > NH4 > Li > Na. 



Subsequent experiments with more dilute CaClo and MgCl2 

 solutions, isosmotic with sea-water, showed that to the former 

 salt practically no responses were given except at the mouth, 

 while to the latter no responses were given save the positive 

 out-pushing of the ctenidia. These bivalent cations are mark- 

 edly less efficient in sensory excitation than are the monovalent 

 alkaline cations. The curious 'positive' reaction of the gills to 

 MgCl2 solutions is similar to that given by the podia of holo- 

 thurians (Crozier, '15 b). 



b. The neutral salts of potassium — KCl, KBr, KNO3, and 

 KI — were compared, at 5/8 M concentration as in the previous 

 tests. The local reactions elicited ^vere very violent in all 

 parts of the body, but in no place were the animals stimu- 

 lated to roll up. The ctenidia responded by contracting until 

 curled in a tight circle against the dorsal wall of the gill cavity; 

 at the same time the side of the foot was moved far a^vay, later- 

 ally, thus exposing the gill cavity more widely, while the girdle 

 was rolled over and inward so as to cover the ctenidia. After a 

 brief interval the foot relaxed. These responses were of variable 

 duration, depending upon the amount of the stimulating solution 

 applied. 



