INTEGUMENTARY PHOTO SENSITIVITY IN A MARINE FISH 265 



greater degree 'of sensitivity than others. This susceptibility to photic 

 stimulation decreases in the following order : head > tail > mid-body 

 regions. 



B. Reactions of blinded fishes 



During the experiments on regional sensitivity, the eyes of the nor- 

 mal fishes employed were usually in comparative darkness. It is im- 

 probable, therefore, that they could have been sufficiently illuminated 

 by diffused light to cause a reaction, even though their partial illumi- 

 nation may have had some influence on it. However, in order to de- 

 termine whether the integument or the eyes were chiefly concerned in 

 such responses as have been described, similar experiments were re- 

 peated on blinded fishes. The behavior of such "physiological" indi- 

 viduals when first placed in the testing-tank was very similar to that 

 of the normal animals. Their period of preliminary uneasiness, how- 

 ever, was slightly longer and they more frequently collided with the 

 walls of the aquarium, since at first they apparently had no idea of the 

 dimensions of their confines. 



1 . To light and dark fields. A single light-field and dark-field experi- 

 ment will be described in detail, in order to show the nature of the re- 

 sponse of blinded fishes to light; there are exhibited several indications 

 of stimulation which precede the true negative response the swim- 

 ming to the darkened end of the aquarium. When a hamlet whose eyes 

 have been enucleated finds itself in that half of the tank which is lighted 

 by a 60-watt lamp, its first visible reaction, the raising of the dorsal fin, 

 occurs in 24 seconds. A few seconds later its chromatophores darken. 6 

 Next it swings its tail toward the light, and finally, after 75 seconds, 

 swims to the darkened half of the aquarium, where it comes to rest. 

 This position is retained for about 3 minutes, whereupon the fish often 

 swims back, more or less completely, into the lighted zone. After a 

 period of relative uneasiness there, which is usually shorter than the 

 interval of quiet in the dark, the negative reaction is repeated. 



Often in such experiments the attempt to return to the light is 

 stopped when the head first becomes illuminated. This momentary 

 hesitation is usually followed by a retreat to the darkened end. Some 

 of the fishes when their heads entered the light, hesitated, then re- 

 versed ends, and finally backed part way into the light, as if to test 



6 Light causes the region of the integument which is illuminated to become 

 darker in color. Sunlight also darkens the color of the skin and shading lightens 

 it. 



