246 LESLIE B. AREY AND W. J. CROZIER 



decide. The comparative distribution of tactile and of osmotic 

 sensitivity is nevertheless suggestive in this connection. We 

 have seen that for stimulation by gentle contact the reactivity 

 of the several regions of chiton's surface was as follows: 



head = ctenidia > edge of foot > girdle > sole of foot; 



while for irritants, such as essential oils, the order of reactiveness 

 for the same parts was: 



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

 > girdle. 



To local osmotic disturbances the sensitivity of these areas ap- 

 peared, on the basis of the experiments with dilutions of sea- water, 

 to be related in the following sequence: 



head > ctenidia = foot > ventral edge of girdle. 



So far as these responses go, they indicate that the receptors 

 concerned in osmotic excitation are distinct from those concerned 

 in tactile reactions, from those concerned with responses to 

 irritants, and (as seen in a following section) from those impli- 

 cated in chemical excitation, but the evidence is not conclusive. 



The sensitivity of the proboscis ('head'), especially of its 

 peripheral edge, is probably concerned in determining the rela- 

 tive immobility of chitons in exposed places at the period of low 

 tide; the same, to a lesser degree, is perhaps true of the edge of 

 the foot. In active creeping the anterior edge of the proboscis 

 is kept in close contact with the substratum, as shown in figure 

 14; this organ undergoes 'spontaneous' local contractive move- 

 ments, depending for their execution on the pressure of fluids 

 contained in its interior spaces (Heath, '05 b) . 



b. The osmotic reactivity of Chiton's soft surfaces is im- 

 portant in connection with the question as to whether sugars 

 are successful as activating agents for this animal. Sea-water of 

 36.5 per mille salinity (5/8 M) has at 27°C. an osmotic pressure 

 of something more than 25 atmospheres, corresponding to a 

 sucrose solution about 0.8 M. The hmits within which various 

 concentrations of sea-water do not stimulate were found to be 

 4/8 to 8/8 M (for the lips, the other regions being less sensitive) ; 

 M/2 sucrose in 5/16 M sea- water gave fair responses from all 



