Sensory Discrimination: The Chemical Sense 8i 



already in motion, it gave no positive response to food in 

 its neighborhood (20). 



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7 



§ 18. The Chemical Sense in Annelids 



In our own experience, as has been said, the "food sense" 

 is represented by the two senses, taste and smell, the stimu- 

 lus for the one being fluid, and that for the other gaseous, 

 so that the latter enables us to perceive objects at a dis- 

 tance. For water-dwelling animals, such as most of those 

 whose behavior we have been describing, the distinction 

 evidently cannot well be drawn. If such an animal per- 

 ceives food at a distance, the stimulus is necessarily dif- 

 fused through the water, and Lloyd Morgan has proposed 

 the term " telaesthetic taste" for the sense which makes 

 such perception possible (504, p. 256). The term indi- 

 cates that this sense corresponds to taste in an air-dwelling 

 animal because the stimulus is fluid, but differs in that it 

 allows perception of a distant object, as taste in the or- 

 dinary sense does not. In the most familiar representative 

 of the Annelida or segmented worms, the common earth- 

 worm, as in the land planarian, a distinction analogous to 

 that between smell and taste in our own sensory experience 

 may be made ; in the leeches and marine annelids it cannot. 



Gentle and continuous mechanical stimulation produces 

 in the earthworm "positive thigmotaxis" ; that is, the ani- 

 mals have a tendency to crawl and lie along the surface of 

 solids (686). That there is some discrimination of edible 

 from inedible substances when in contact with the body 

 Darwin thought probable from the apparent preference 

 of the worm for certain kinds of food (171). In the earth- 

 worm Allolobophora fatida we find a differentiated response 

 to contact .and-febe mical stimulation. These worms live 



