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LESLIE B. AREY AND W. J, CROZIER 



For purposes of comparison, 'reaction times' were measured, 

 covering the interval from the contraction of the gills until the 

 moment they were judged to be completely relaxed. 



The results obtained in this way may be illustrated by the 

 following sets of data: 



KCl was plainly more stimulating than the other salts. After 

 numerous attempts to graduate the other three under various 

 conditions as to quantity of solution and quickness of application, 

 in the same and in different animals, the series 



KCl > KNO3 > KBr > KI 



was chosen as the most satisfactory. 



c. Successive dilutions of several representative substances 

 were employed to stimulate various regions of chiton's surface, 

 with the object of establishing the respective limiting dilutions 

 effective in excitation. Sea-water was largely used as the sol- 

 vent in these tests, since it is the normal fluid medium for 

 chiton; moreover, as we shall see presently, the surface of the 

 animal is reactive locally to osmotic conditions differing from 

 the normal. 



KCl: To KCl solutions more concentrated than N/16, made up in 

 sea-water, all of the ventral portions of Chiton are reactive. 



N/16 All portions respond except the girdle. 

 N/32 No response from the sole of the foot. 

 N/48 Good reactions from the mouth region ; the gills some- 

 times fail to react. 

 N/64 Gills fail to respond. 

 N/80 Mouth response good; edge of the foot weak. 



