INTEGUMENTARY PHOTOSENSITIVITY IN A MARINE FISH 269 



100 per cent (from 275 to 562 seconds). Stated in terms of the Bunsen- 

 Roscoe law (product of intensity into time of exposure) the agree- 

 ment is not very close (27,500 and 33,720), but still it suggests that 

 exhaustion, like stimulation, may be a photo-chemical process. 9 



D. Location of the photoreceptors 



Payne ('07, pp. 320-321) in endeavoring to explain the fact that, in 

 Amblyopsis, the negative phototropism is greater when the fish is il- 

 luminated from above than when from the side, has suggested that 

 brain and spinal cord may be " affected directly on account of the trans- 

 parency of the tissues above them." This is equivalent to suggesting 

 that the photoreceptors may be located in the brain and spinal cord; 

 but Parker ('05) has contended that in Ammocoetes the receptors are 

 contained in the integument, for he was able to stimulate this fish by 

 directing light against its belly, which, he argued, contained viscera 

 impenetrable to the light which he employed. He had previously 

 ('03, p. 33) shown that the brain and spinal cord of frogs are also in- 

 sensitive to light. 



In view of this difference of opinion an attempt was made to de- 

 termine the location of the sensory elements in blinded hamlet-. For 

 this purpose the capacity of light (from a 100- watt bulb) for stimulating 

 the integument after it had passed through a considerable mass of 

 muscular tissues was tested. The apparatus used for this experiment 

 consisted merely of a piece of the lateral body muscles, about half 

 an inch in thickness, from another hamlet, held in front of the aperture 



9 An incidental question the relative fatiguing effects of continuous and of 

 very slowly intermittent light was also considered. The data of five experi- 

 ments, of 16-20 exposures each, indicate that continuous illumination and suc- 

 cessive illuminations of the same intensity which are separated from each other 

 by one-half or one minute intervals, are about equal in their power to effect 

 complete exhaustion of the cutaneous photoreceptors. The experiments, how- 

 ever, are too fragmentary and the results too inconstant to warrant any general 

 conclusions on this point. If after many tests this relationship should prove to 

 be constant, the difference between it (although there is no ground for assuming 

 a parallelism) and the results of Parker and Patten ('12) is perhaps explained by 

 the fact that a minute, or even one-half minute, interval between exposures is 

 sufficient for the partial recovery of the sense organs. I was ignorant, at the 

 time of this experimentation, of the work of Parker and Patten; otherwise I 

 might have undertaken, as they did, experiments with light which was interrupted 

 at intervals of high frequency. It would be of value to compare from this point 

 of view the photoreceptors of the eye and skin. 



