THE SENSORY RESPONSES OF CHITON 245 



rated extract in sea-water, induced good reactions from all parts, 

 including the girdle, which reacted by bending and twisting 

 away. 



The order of sensitivity of the several parts of the surface was 

 made out to be: 



gills > head > edge and posterior end of foot > sole of foot > 

 ventral girdle surface > dorsal girdle surface. 



e. Osmotic excitation of the general ventral surface of Chiton 

 was investigated by means of dilutions of sea-water. 



Three parts sea-water + 1 -part rain-water: gave responses only upon 

 the lips. 



Two parts sea-water + 1 part rain-water: no response from the 

 mantle edge, but other parts react weakly. 



One part sea-water + 1 part rain-water: no response from the mantle 

 edge, good responses from other parts. 



Rain-water: sole of the foot puckers away strongly; gill response 

 very active; even the girdle reacts, and the shell tends to roll up. 



Sea-water concentrated by evaporation to one half its original volume 

 gave good reactions on all regions. 



The same upper limit of osmotic stimulation appears in the 

 effects of glycerin solutions: 



Glycerin: in sea-water solution, 



5M Strong reactions everywhere, even from the mantle edge. 

 3M Mantle edge fails; pitting of the foot away from the fluid 



is deep and local. 

 2M Same, but weaker responses. 



IM Ctenidia give a faint response, but only occasionally; 

 mouth region still sensitive; very doubtful responses from 

 the foot. 

 M/2 Faint response from the lips; ctenidia doubtful. 



2. The mode of excitation by solutions 



The observations above detailed require analysis from several 

 points of view. We shall deal first, briefly, with the evidence 

 they contain relative to the method of activation by solutions. 



a. It is clear that 'osmotic sensitivity' is possessed by the 

 soft superficial tissues of chiton. Whether this depends upon 

 the activation of 'tactile' or other end organs or upon the stimu- 

 lation of chemoreceptors proper we cannot at first entirely 



THE JOUBNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 2 



