FLAT-FISH. 441 



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 colour, it was 

 thought highly probable that the same might be the case with regard to flat-fish, 

 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 experiment 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 colorisation 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 flat-fish' 

 are met with, that is specimens in which both sides are coloured, and one eye 

 situated on the edge of the head ; such monstrosities having been observed 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 coloured 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 actually seen 

 a turbot so doing. But Dr. Cunningham writes that there is "no satisfactory 

 evidence at present that the monstrous specimens, whose metamorphosis is per- 

 manently arrested, swim about, in whatever positiori, any more frequently, or rest 

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

 tion 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 coloured like the upper, the anterior fourth being white. This specimen, 

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

 induced to leave the bottom long enough to enable me to see whether it holds 

 itself perfectly horizontal or not. I have never seen it leave the bottom of its 

 own accord ; it lies always buried in the sand up to its eyes, and, when disturbed, 

 makes violent struggles to bury itself again." 



When lying on the sandy bottom of the sea and they prefer sandy to muddy 

 situations^flat-fish are almost indistinguishable from their inanimate surroundings, 

 the spots with which the bodies of many of them are marked harmonising exactly 

 with the bright-coloured pebbles strewing the sand. This resemblance is, perhaps, 

 carried to the fullest extent in the flounder, as anyone who visits a large aquarium 

 may ascertain for himself. Occasionally rising to the surface, they swim with an 

 undulating lateral movement of the body, which is decidedly graceful ; and they 

 are found in shallow water, or at moderate depths. They are inhabitants of all 

 seas, except those of the polar regions, and where the coast is precipitous and 

 rocky; and although more numerous in the tropics, they attain their greatest 

 development in point of size in the temperate regions. Many species, such as 

 flounders, ascend rivers to a considerable distance ; and a few have become 

 accustomed to a fresh-water existence. As regards food, the whole of the species 



