THE FLATFISHES 2813 



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 ordi- 

 nary 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 flatfish are 

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

 called silvery layer is present on both. Young flatfish, 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; but 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 of these fish 

 which are of sufficient interest to merit a brief notice. Knowing, as we do, that 

 among plants absence of light leads to the deprivation of color, it was thought highly 

 probable that the same might be the case with regard to flatfish, more especially 

 since the absence of coloration in the olm among the Amphibians is clearly due to 

 the same cause. To test this, flounders were kept in a tank, in which, by the help 

 of a mirror, light was so introduced as to give to the fishes the unwonted experience 

 of illumination ascending from below instead of coming down from above. The experi- 

 ment was conducted for several months; some of the flounders died, in others no great 

 effect was produced, but in some cases the white surface became marked over with 

 pigment. This experiment demonstrates that the capacity for colorization existed in 

 the skin, but that light was wanted to call it into action; and it may also be inferred 

 that the incidence of light must in general be the reason why the upper surfaces of 

 animals are more strongly pigmented than the lower. Occasionally what are known 

 as "double flatfish" are met with, that is specimens in which both sides are colored, 

 and one eye situated on the edge of the head; such monstrosities having been ob- 

 served in the turbot, flounder, plaice, sole, etc. In one turbot the right eye was on 

 the edge of the head, so that the dorsal fin, instead of extending, as usual, to the 

 front of the head, was separated therefrom by a concavity, the right side being col- 

 ored like the left, although somewhat less strongly. It has been thought that these 

 " double fish " swam about in a vertical position, M. Giard stating that he has actu- 

 ally seen a turbot so doing. But Dr. Cunningham writes that there is "no satis- 

 factory evidence at present that the monstrous specimens, whose metamorphosis is 

 permanently arrested, swim about, in whatever position, any more frequently, or 

 rest on the ground less constantly than their normal brethren. I have under ob- 

 servation a living double specimen of the plaice; its eye is on the edge of the head, 

 the dorsal fin terminates behind the eye, and the posterior three-fourths of the lower 

 side are colored like the upper, the anterior fourth being white. This specimen, 

 instead of showing a tendency to continue swimming in the water, cannot even be 



