82 The Animal Mind 



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 stimulus 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 (686). Using the human terms, the case is one 

 of taste rather than smell. Nagel suggests that the earth- 

 worm's chief use for a chemical sense is to help it find the 

 moisture which is necessary to its life (522) ; but curiously 

 enough Allolobophora fceiida 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 

 (686). Parker and Parshley (555) find that the head 

 end of the worm is negatively stimulated by contact with 

 a dry surface, and will withdraw soon after such contact. 

 There seems no satisfactory evidence that worms respond 

 to chemical stimulation from a distance by positive re- 

 actions, although Darwin believed that they found buried 

 food by "the sense of smell" (171). Chemical stimuli 

 not in contact with the body do produce negative reactions 

 (522), but these reactions do not differ from the responses 

 to strong mechanical stimulation. They are of various 

 forms — turning aside, withdrawing into the burrow if 

 the tail is already inserted, squirming, and so on, the dif- 

 ferences being correlated with differences in the intensity 

 and location of the stimulus and in the excitability (physi- 

 ological condition) of the animal. But nothing in the 

 character of the response suggests that negative reaction 



