THE SALMON FAMILY 83 



these iridocytes are present as a continuous reflecting 

 surface in the deep layers of the skin, the light reflected 

 by this surface is seen through the skin, and gives the 

 fish a dull white appearance, but when, in addition, 

 iridocytes are present outside the scales, as I have 

 already explained, they act like so many prisms and, 

 breaking up the light reflected from the deep reflecting 

 surface, give the fish its silvery iridescence. These irido- 

 cytes consist of guanin, and guanin is derived from rich 

 animal food such as the cyclops and daphnia in the 

 second pond. The peculiar pink shade that these fish 

 had acquired was due to the partial masking of the 

 normal colour cells in the trout by the excessive number 

 of iridocytes which had been formed on the scales of the 

 fish. 



When the brown trout acquires the habit of going 

 to sea, he has an opportunity of feeding on abundance 

 of guanin-forming food, and acquires a similar silvery 

 appearance. In passing, I would draw attention to the 

 fact that this alteration in the appearance of the trout 

 in the sea helps to protect him by making him incon- 

 spicuous, for though the dark colour and the numerous 

 brilliant spots make him inconspicuous on the pebbly 

 bottom of a stream, they would attract attention in the 

 sea. The increase of the iridocytes in his skin make him 

 reflect his surroundings in a similar manner to the dace 

 which has been already described. 



Further, though the bright spots of the brown trout 

 all disappear in the sea, he is not altogether devoid of 



