Sensory Discrimination: The Chemical Sense 85 



feelers could distinguish perfume. Taste he found best 

 developed near the lips, and touch sensibility distributed 

 over the body, but especially toward the end of the feelers 

 (834, 835). 



Of two freshwater snails, Physa and Lymnaea, the latter, 

 whose movements are slower, can sense food at a greater 

 distance than the former. In Physa an interesting rela- 

 tion between the chemical and mechanical stimulation 

 produced by contact with food is apparent. "If Physa," 

 says Dawson (177), "was moving at a moderately rapid 

 rate when it came in contact with the meat, it received a 

 sufficiently strong stimulus to cause it to turn away, to 

 pause and then turn back. It would seem that the me- 

 chanical stimulus was not only sensed first but obeyed, 

 and then the chemical stimulus was in turn sensed and 

 obeyed." The limpets Patella and Calyptraea respond 

 to the neighborhood of non-irritating oils by withdrawing 

 reactions (583). Irritating chemicals, of course, are not 

 proper olfactory stimuli, but one can hardly be sure that a 

 stimulus which like oil of bergamot would be non-irritating 

 to the human mucous membrane, is non-irritating also to 

 the body surface of an animal. Mollusks in general 

 seem to have chemical sensitivity distributed all over the 

 body surface, although certain regions are especially sensi- 

 tive. Pieron (585) finds in marine snails three modes of 

 chemical excitability: an aerial distance excitability, on 

 all parts of the body with predominance of the mouth, 

 the anterior edge of the foot, and the siphon ; a contact 

 sensibility in both air and water, on the mouth, the 

 horns, and probably elsewhere; and a delicate distance 

 sensibility in the water, located in the regions of the 

 mouth, the horns, the anterior edge of the foot, and 

 the osphradial region. 



