SENSOEY REACTIONS OF CHROMODORIS ZEBRA 267 



II. MECHANICAL EXCITATION 



1. Tactile stimulation 



The oral tentacles are very sensitive to touch, especially at 

 the tip. When the tip alone is very lightly touched with a fine 

 glass hair, it is contracted and slightly introverted. To shghtly 

 more intense stimulation, however, and always when touched at 

 the side or at the base, the tentacle is introverted at the base 

 after the fashion of a glove-finger. To unilateral stimulation of 

 one tentacle, even to sharp and repeated touches, that tentacle 

 alone responds. But after a tentacle has been completely, ex- 

 cessively, contracted, strong continued local mechanical stimula- 

 tion of it (while remaining retracted) causes the opposite tentacle 

 to be retracted. In this case the whole head region is more or 

 less contracted, and it may be that the skin at the base of the 

 retracted tentacle must be stimulated in order to result in a 

 spreading of the response to the other one. 



The 'rhinophore' on the same side with a stimulated tentacle 

 usually contracts slightly, by a twitch of the muscles at its base, 

 synchronously with the activated tentacle itself. If the stimu- 

 lation is originally strong or if it is repeated, the opposite 'rhino- 

 phore' also responds, but usually to a less degree. Stimulation 

 of a tentacle also involves response from the head region gener- 

 ally, causing it to retract; at the same time the buccal veil is 

 drawn down so as to cover the whole mouth region, including the 

 anterior edge of the foot. The anterior end of the body is 

 under these circumstances contracted more strongly on the stimu- 

 lated side, and after reextension the whole body is usually caused 

 to bend in the opposite direction, away from the side originally 

 stimulated. If the anterior part of the foot should not be in 

 contact with the substratum, it also contracts, on the homolateral 

 side, when a tentacle is touched. This general form of reaction 

 is the common response when the nudibranch is stimulated any- 

 where with sufficient severity. Further evidence for the neuro- 

 muscular unity of the head region will be found in what follows. 

 To a single light touch upon a tentacle, the general head re- 

 sponse is very sHght, but is nevertheless evident. The full head 



