INTEGUMENTARY PHOTOSENSITIVITY IN A MARINE FISH 267 



to the sensitivity of the three regions, the order of the retardation being : 

 head, mid-body, tail; that of sensitivity in both normal and blinded 

 fishes: head, tail, mid-body. The explanation of this order of retarda- 

 tion is doubtless to be sought in the fact that it was impossible com- 

 pletely to screen the head when the mid-body and tail regions were ex- 

 posed to light, with the result that the head (and consequently the eyes) 

 were more highly stimulated by mid-body than by tail illumination. 



The reaction times of the integument of normal and blinded fishes in 

 different regions can not be regarded as wholly satisfactory, for the 

 reason that, as above stated, the illumination of each region involved a 

 greater or less illumination of the other regions. While, therefore, 

 they show clearly which is the most and which the least sensitive area, 

 they may not accurately represent the degree of sensitivity. 



By way of summary, then, it may be said: first, that the integument 

 of Epinephelus striatus exhibits a negative photosensitivity, which is 

 independent of the eyes ; secondly, that it also shows, fairly constantly, 

 three regions of the integument which manifest this sensitivity in dif- 

 ferent degree as follows : head > tail > mid-body. 



3. Photodynamic nature of the reponses. As noticed in most of the 

 preceding experiments, the quickness of the response depends rather 

 closely upon the intensity of stimulation. An increase in the wattage 

 usually decreased the required period of stimulation and vice versa. 

 This suggests that the responses may be photodynamic in nature. 



Another set of experiments was, accordingly, undertaken for the pur- 

 pose of testing the validity of this assumption. To do this the same 

 local region, or the skin of one whole side, was stimulated successively 

 with different intensities of light and the time of reaction carefully noted. 

 In order to minimize the error which might be caused by exhaustion, 

 care was taken to use alternately high and low intensities and especially 

 to allow ample intervals for recovery. The results, some of which are 

 given further on, substantiate the conclusions derived from the earlier 

 observations, establishing the fact that the reactions of the hamlet to 

 photic stimulation of the integument are dynamic in character. 



The period of illumination required to produce a response in these 

 experiments was found (as the result of about twenty-five tests at each 

 wattage) to be for 100-,60- and 40-watt lamps 24, 35, and 53 seconds, 

 respectively. In other words a decrease of 40 per cent from the high- 

 est intensity of the light (from 100 to 60 watts) resulted in an increase 

 of about 45 per cent in the reaction time (from 24 to 35 seconds) , and a 

 60 per cent decrease in the brightness of illumination (from 100 to 40 



