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 eyclops 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 
