chap, xix.] FLAT-FISHES. 467 



The converse variation, by which the lower side assumes the 

 colour of the upper side is important in several aspects. 



Interest has of late been drawn to this subject especially through an experiment 

 recently made by Cunningham 1 , who found that of a number of young flat-fishes 

 reared in a vessel illuminated by mirrors from below, some became partially marked 

 with pigmented patches on the lower side. The suggestion was made that this 

 pigmentation was induced by the direct action of light. It is of course impossible 

 here to enter into the theoretical questions raised in connexion with this subject 

 and this account will be confined to description of the colour- variation as seen in 

 nature and of the singular variation in structure commonly associated with it. 

 Mr Cunningham has obligingly advised me in connexion with this subject. 



Pigmentation of the lower side has been seen in Rhombus 

 maocimus, R. Icevis, Pleuronectes flesus, P. platessa, P. oblongus, 

 Solea vulgaris [?] and probably other forms. Attention is drawn 

 to one feature in these changes which from our standpoint has an 

 important bearing. When the underside of a flat-fish is pigmented, 

 it is often not merely pigmented in an indefinite way but it is 

 coloured and marked just as the upper side is. There are, I know, 

 many specimens upon whose undersides a brownish yellow tint is 

 either generally diffused or restricted to patches, but when there 

 is pigment of a deeper shade, as in all the well marked cases of 

 the variation, the colour and markings are closely like those of the 

 upper side. For example, a Plaice (P. platessa) sent to me by Mr 

 Dunn of Mevagissey is fully coloured over the posterior half of the 

 lower side ; but there is not merely a general pigmentation, for the 

 coloured part of the lower side is marked ivith orange markings 

 exactly like those of the upper side. 



More than this : it was found by passing pins vertically through 

 the body that there was in the case of most of the spots a close 

 correspondence in position behveen those of the upper and those of 

 the loiver side. There were 13 spots on the coloured part of the 

 lower side, which extended slightly beyond the line of greatest 

 width. Of these, 13 spots on body and fins coincided exactly with 

 those of the upper side ; 2 coincided nearly ; 2 were not repres- 

 ented on the upper side ; and 2 spots of the upper side were not 

 represented on the lower. From these facts it is clear that in 

 " double " flat-fishes we have an instance of symmetrical variation 

 of one half of the body into more or less complete likeness of the 

 other half, resembling other cases of Homceosis in Bilateral Series 

 already noticed. 



This is made the more evident by the fact that in the two best 

 described specimens of "double" Turbot (No. 727) not merely did the 

 lower side resemble the upper side in point of colour, but upon it were 

 also present the bony tubercles normally proper to the dark side, being 

 only slightly less well developed on the lower side than on the upper. 



succeeded in seeing an entirely white specimen, though individuals partially white 

 on the upper side are not rare. See also Zool, pp. 4596, -1914. Zeugopterus puncta- 

 tus white on both sides, Day, Brit. Fishes, n. p. 19. 

 1 Cunningham, J. T., Proc. Roy. Soc, 1893. 



30—2 



