440 SOFT-PINNED GROUP. 



to one side of the body (in some cases the right, and in others the left), the flat- 

 fishes differ not only from all other members of their class, but likewise from all 

 other vertebrates. The body is strongly compressed and flattened, with the side 

 which is turned upwards, and on which are situated the eyes, coloured dark, while 

 the opposite, or eyeless side is, as a rule, colourless. The bones of the head are 

 unequally developed and unsymmetrical ; and the dorsal and anal fins are of great 

 length, and undivided, the former often extending forwards so as to separate the 

 blind from the eyed side of the head. In the most specialised forms the teeth and 

 jaws are more developed on the lower or blind side than on the other, and there is 

 no air-bladder. Dr. Cunningham, who has paid special attention to the structure 

 of these fishes, writes that " mere dissection of adult specimens shows that the 

 anomalous position of the eyes is due to a distortion of the facial region of the 

 skull. The cranial region of the skull is but slightly altered, but the interorbital 

 parts of the two frontal bones are bent away from their original position in the 

 dorsal median line down to the side of the head, and they are also compressed into 

 a thin plate. But the eyes have pretty nearly the same relations to the inter- 

 orbital septum as in an ordinary fish. There is one eye on each side of the septum 

 as usual. It is, in fact, the curious condition of the dorsal fin in the flat-fish, even 

 more than the mere distortion of the eyes, which makes it so different from the 

 ordinary fish. If the fin terminated some distance behind the eyes, or if it was 

 prolonged in the direction it ought to follow, that is along the line which divides 

 the two frontal bones from one another, it would be plain at a glance which was 

 the left side of the head and which the right. It would then be obvious that the 

 left eye was still on the left side of the head, and the right eye on the right. But 

 the dorsal fin does neither of these things. The external ethmoid bone belonging 

 to the blind side is much enlarged, and sends back a process outside the eye 

 belonging to that side to meet another process from the cranial region of the skull. 

 Thus the eye which has migrated the upper eye when the fish is held in a vertical 

 plane is enclosed in a complete bony orbit, while the lower eye is merely bounded 

 on its outer side by the jaw muscles. It is on this bony bridge, entirely foreign 

 to the anatomy of an ordinary fish, that the dorsal fin supports itself in its 

 advance towards the snout. Properly speaking, the left side of the face in a 

 plaice, for instance, extends from the ventral edge, or chin, to the line between 

 the eyes, but the dorsal fin in its anterior extension divides this side of the face 

 into two parts." 



The pigment-bearing elements in the coloration of the dark side of flat-fish 

 are known as chromatophores ; and while these are absent from the light side, the 

 so-called silvery layer is present on both. Young flat-fish, which are generally 

 met with in the open sea, are transparent and perfectly symmetrical, with one eye 

 on each side of the head, and swim in the vertical plane like ordinary fishes. 

 That flat-fishes have originated from symmetrical ancestors is quite evident, their 

 individual metamorphosis indicating the manner in which the evolution took 

 place. As to the inducing causes of this evolution and metamorphosis, there is still 

 some difference of opinion ; and as it is a subject which does not come within the 

 province of this work, it need not be further alluded to. There are, however, certain 

 experiments with regard to the normal absence of coloration on the under surface 



