Sensory Discrimination: the Chemical Sense 79 



and chemical stimulation. These worms live in barnyard 

 manure. When placed on scraps of shredded filter paper 

 moistened with water they refuse to burrow; when the filter 

 paper is wet with a decoction of the manure they burrow as 

 soon as they come into contact with it. The adequate stimu- 

 lus for burrowing is thus a combined mechanical and chemical 

 one; the chemical stimulus alone is insufficient, for filter 

 paper thus prepared has no effect on the worms unless they 

 are actually in contact with it (387). Using the human terms, 

 the case is one of taste rather than smell. Nagel suggests 

 that the earthworm's chief use for a chemical sense is to help 

 it find the moisture which is necessary to its life (292) ; but 

 curiously enough Allolobophora fatida seems to have no power 

 of doing this from a distance. Smith found that a worm 

 would crawl around a wet spot on paper until its skin dried, 

 without crawling into it. If by accident it happened to touch 

 the moist place, it would enter and remain there (387). There 

 seems no satisfactory evidence that worms respond to chemicaL 

 stimulation from a distance by positive reactions, although/ 

 Darwin believed that they found buried food by "the sense 

 of smell" (91). Chemical stimuli not in contact with the 

 body do produce negative reactions (292), but these reactions X 

 do not differ from the responses to strong mechanical stimu- 

 lation. They are of various forms turning aside, with- 

 drawing into the burrow if the tail is already inserted, squirm- 

 ing, and so on, the differences being correlated with differences 

 in the intensity and location of the stimulus and in the excita- 

 bility (physiological condition) of the animal. But nothing 

 in the character of the response suggests that negative reaction 

 to a chemical stimulus has a different conscious accompani- 

 ment from that of negative response to a mechanical stimulus. 

 The most natural interpretation of them all on the psychic 

 side is that of unpleasantness, increasing in intensity as the X 



