FLAT-FISH ; 
441 
of these tish 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. I 11 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 position, 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 
