THE SENSORY RESPONSES OF CHITON 255 



less, it is true that in those instances where photoreceptors, for 

 example, may be isolated and distinguished, we frequently find 

 that the sensory elements are removed from the external surface 

 of the animal, protected from the action of environmental chemi- 

 cal disturbances and deforming contacts. From this standpoint 

 one of the factors determining differential receptivity is to be 

 found in the degree of anatomical isolation of the receptor; 

 another, in the morphology of the sensory cell or organ, as in the 

 development of distal projections. These factors of form and 

 position undoubtedly facilitate the respective operation of dif- 

 ferent qualities of stimulation, and to that extent determine the 

 functioning of differential irritabihty. But the problem is not 

 in this way wholly solved. The evidence for 'generalized sense 

 organs' is in some cases good, though perhaps not final. Even 

 on the sole of the foot of Chiton physiological evidence of sen- 

 sory separateness for photo-, tacto-, and chemo-reception is 

 available. The skin of Amphioxus is fully as sensitive as that 

 of other marine animals to touch and to chemical influences 

 (Parker, '08), but is insensitive to light. The additional factor 

 is probably found in the possession by certain receptor cells of 

 special substances which enter into excitation reactions. 



Even if in some cases it could be shown that the epithelial 

 cells of an animal were open to sensory activation by a variety 

 of means, it would not lead to the view that a 'universal' type of 

 sense organ (Nagel) was that first developed in evolution, re- 

 ceptors of special kinds being by some obscure metaphysical proc- 

 ess subsequently derived from it. In coelenterates, so far as we 

 know (Parker, '17 a), tactile receptors and chemoreceptors are 

 organically distinct. 



c. The reactions of Chiton to local stimulation are of a character 

 consistent with the known distribution of the central nervous 

 system. At the sides of the body, those parts innervated by the 

 pallial strands are conspicuously homolateral in their responses. 

 The coordination of the pedal musculature for the production of 

 locomotor waves depends upon a mechanism locally contained, 

 and apparently upon the integrity of the pedal cross-connectives. 

 The coordination of the gill movements on one side of the body 



