COLORATION OF THE SKINS OF FLAT-FISHES. 115 
on the inner surface of the skin there occurs another layer of chroma- 
tophores, black and yellow as in the superficial layer. Associated 
with the chromatophores here there are no iridocytes, but the place 
of the latter is taken by a continuous layer of opaque reflecting 
substance similar to that of which the iridocytes are composed. On 
the lower side of the fish chromatophores are entirely absent, but the 
iridocytes of the superficial layer are well developed. It is not these, 
however, which cause the opaque whiteness of the lower side of the 
flounder, for the layer containing these can be removed with a razor, 
and the whiteness of the skin remains. ‘This characteristic of the 
lower skin is due to a thick, dense, continuous layer of reflecting 
substance on the inner surface of the skin, corresponding to the layer 
mentioned above in similar position on the upper side. This layer is 
much thicker on the lower side than on the upper. 
The character and location of the elements of coloration are quite 
similar in other flat-fishes, but they are not always developed to the 
same degree. The chief variation is in the subcutaneous reflecting 
layer, which is in some species, e.g. the Megrim (Arnoglossus 
megastoma), almost entirely absent, represented only, whether on the 
upper or lower side, by separate small plates quite similar to iridocytes, 
but not so regularly arranged. In fact, comparative observations 
of different species, and the history of the development in the 
flounder, prove that the internal reflecting layer is actually derived 
from a layer of separate iridocytes which enlarge until they become 
continuous. ‘This explains why the lower skin in the young flounder 
