The Reversal of Tropisms 113 



of the opposite side of the body, but by an actual 

 lengthening of the stimulated side. This lengthen- 

 ing is probably effected by the contraction of the 

 dorso-ventral muscles which run from the dorsal to 

 the ventral body wall, and possibly also by the cir- 

 cular muscles. The response is local and near the 

 stimulated area as it generally is in the lower inver- 

 tebrates. The positive and the negative reactions 

 depend either upon the functioning of two quite dis- 

 tinct reflex arcs, or, In case the afferent impulses 

 travelled in the same path, upon the diversion of 

 the stronger stimulus Into a new pathway. It is 

 well known that strong stimuli often break over 

 into additional pathways of discharge, and it is of 

 course possible that such a phenomenon might occa- 

 sion a reversal of reaction. 



What part the inhibitory influence of stimuli may 

 play in reversal is uncertain. Loeb has alluded to 

 the possible role of this factor in one of his earlier 

 papers ('93), but without developing the suggestion 

 further. When an earthworm turns away from the 

 light we may assume, as in fact has been done by 

 Davenport ('97), that the action of the muscles on 

 the more illuminated side is inhibited, and that the 

 animal accordingly turns away from the stimulus. 

 There is no evidence, however, that such an inhibi- 

 tory process occurs. A worm turns violently away 

 when one side is touched with a hot needle, but the 

 movement is not due to the mere inhibitory effect 

 of the stimulus. If the turning away from the hot 



