THE SENSOEY RESPONSES OF CHITON 199 



tactile irritation is distributed upon the body of Chiton in a 

 manner appropriately correlated with its structure and habits. 

 The various responses obtained from contact with a small sur- 

 face are such as would have a protective influence. Reference 

 should be made at this point to the local closing together of the 

 dorsal valves when the intertegmental mantle is irritated, and 

 especially to the ventralward movement of the girdle, associated 

 with local retraction of the ctenidia and a corresponding local 

 movement of the foot, which follows a touch upon the dorsal or 

 ventral surface of the girdle. The preservation of the ctenidia 

 from injury, and more particularly the effective use of the girdle 

 for the exclusion of foreign objects and as a hold-fast, are de- 

 pendent upon responses such as we find the girdle to exhibit. 

 Further detailed correlations of this character might be pointed 

 out, but enough has been said to indicate the useful nature of the 

 responses. The 'rolling-up' reaction has, when carried to com- 

 pletion, a clearly 'purposeful' aspect, as already intimated. Yet 

 the natural history of Chiton yields no evidence that this re- 

 sponse is ever used. We consider that it is the inevitable out- 

 come of maximal possible contraction in the chiton's effort to 

 produce suction, and that it is neither of specific protective sig- 

 nificance nor of the nature of a ' reflex. ' Confirmation of this view 

 is found in the fact that sometimes a 'rolled-up' chiton will re- 

 main for hours tightly curled, although placed in position pur- 

 posely made favorable for reattachment should it unroll. On 

 the other hand, after a short time upon its back, a chiton may 

 spontaneously uncoil itself and remain fully exposed for a long 

 time, if unstimulated. Moreover, isolated parts of the animal 

 give (or attempt to give, so far as their deficiencies permit) the 

 'rolling-up' response when they are activated. 



D. The tactile receptors. The superficial layer of the shell 

 plates of chitons is traversed by numerous canals, occupied by 

 specialized organs having the histological appearance of sensory 

 receptors. These canals are more or less nearly perpendicular 

 to the surface (at least peripherally), and the organs they con- 

 tain, piercing the tegmentum, are described as projecting slightly 

 beyond its general surface. The remarkable character of these 



