440 
SO FT-FINNED 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 
