THE SENSORY RESPONSES OF CHITON 233 



3. Distributio7i and nature of photoreceptors 



a. For illumination. We have described thus far the responses 

 obtained from chitons submitted to photic excitation involving 

 the edge of the girdle, a large portion of the surface, or the whole 

 surface of the animal. There remain to be considered the dis- 

 tribution and variety of the photoreceptors. 



A chiton from which the girdle has been completely removed 

 is still oriented by light. This fact, taken together with the 

 known histological structure of the sensory organs in the shell 

 plates, suggested that the surface of the shell might contain 

 photosensitive organs. Chitons were rigidly clamped in the way 

 already described in discussing tactile stimulation (p. 200), and 

 allowed to write their contractions kymographically. Tested in 

 air, the shell was found to be sensitive to moderate faradization. 

 When so stimulated, the animal undergoes a pronounced general 

 'curling up,' which ceases when the current is interrupted. A 

 similar result follows the application of a spot of light. ^^ By this 

 method it can also be shown that the shell plates are sensitive to 

 shading. They do not appear sensitive to increased illumination. 



These results, together with those obtained in orientation 

 experiments, furnish adequate proof that the 'shell eyes' of 

 chiton are indeed sensitive to light. We did not attempt to 

 localize the minimal areas sensitive to light, but the illumination 

 of one-eighth of the surface of an anterior or posterior valve with 

 an intense spot of light is sufficient to induce a good response. 



In the genus Chiton 'extrapigmental' eyes are lacking (Moseley, 

 '85; Plate, '99; Nowikoff, '09), the 'intrapigmental' eyes being, 

 however, plentifully distributed over the surfaces of the valves 

 according to a definite pattern; their locations are well shown in 

 eroded shells. They are most abundant in the anterior and 

 posterior valves, but occur on all the shell plates. 



The 'eyes' are not the only parts sensitive to light, however; 

 the ventral edge and the dorsal surface of the girdle indicate by 



^^ This method of investigation permits quantitative work of a relatively 

 precise kind; but in the present paper we are concerned mainly with qualitative 

 effects. 



