X. On the coloration of the skins of fishes, especially of pleuronectidæ

The investigations which form the subject of this paper were originally commenced with the object of obtaining some reliable definite evidence on the inevitable question, why is the lower side of a Flat Fish white and the upper side coloured? The answer given to the question necessarily involves an opinion on a great many others, and hitherto the terms of the answer have depended rather on the general views held by those who gave it than on a special investigation of the phenomenon. Conversely it was therefore to be expected that a special investigation of the question would have some importance as a test of the truth of conflicting general views and theories. In order to ascertain how far the action of light was capable of producing pigment on the lower sides of Flat Fishes, a series of experiments has been carried out by Mr. Cunningham, in continuation of one of which an account was published by him in 1891. But a review of the morphological and chemical relations of the elements and substances of the coloration being necessary for the discussion of these experiments and their results, we have thought it advisable to place this part of the subject first. For Sections III. and IV. of the memoir Mr. Cunningham is alone responsible, and Section II. is almost entirely the work of Dr. MacMunn. Both authors have contributed to Section I. The subjects of the various sections are the following:— I. Structure and Relations of the Tissue-elements, to which Coloration is due— a . Comparative Anatomy, b . Histology, c . Development.