Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
SPOTTED SOLE.
A larger and coarser fish than the common sole.
Photo by W. Saville-Kent, F.Z.S.] [Milford-on-Sea.
HALIBUT.
The largest member of the Flat-fish Family. The back-fin usually commences much farther forward.
Flat-fishes may be reckoned among the most important of food-fishes, and besides are of quite peculiar interest, on account of the remarkable modifications of structure which they have undergone. They differ from all other vertebrates in that, save for the first few weeks of existence, they spend the whole of their lives with one side of the body uppermost—the right or left, according to the species. Whether resting or swimming, this position holds good. The newly hatched fish, however, maintains the normal poise of the body, the back being uppermost. Of the many changes which the organs of the body undergo during this strange transformation from a "round" to a "flat" fish, one of the most interesting is that which concerns the eyes. These, in the very young fish, lie one on either side of the head; but as the fish grows older it begins to lie on its side on the ground, and ultimately, when it is two or three months old, loses the power of sustaining itself in an upright position altogether. The most remarkable feature in this very strange mode of development is, that as the fish comes to lie more and more on its side, so the eye which is undermost begins to move round to the other side, till eventually the two eyes lie side by side on the upper-surface. Strangely enough, in some species the eye moves round the head, passing over its edge, and so to its place beside the stationary eye, whilst in others it acquires its ultimate position by moving through the head, sinking in on one side and appearing again on the other. The coloration of these fishes is also peculiar, in that the two sides are quite differently coloured, the upper side resembling in tone that of the sea-bottom, whilst the under side is pure white. In the young fish, before the habit of lying on one side has been acquired, both sides are coloured alike. The difference in coloration between the two sides of the adult fish appears to be due to the effect of light, since in flat-fishes kept in a tank with a mirror at the bottom the under-surface was found in many cases to be very largely coloured. But the colour of the upper-surface is by no means constant. On the contrary, the flat-fish appears to possess the power, in common with all other fishes, of changing its colour so as to harmonise with the tone of its surroundings. Thus on a light sandy ground the exposed surface of the body becomes pale, while on a dark muddy bottom it is almost black. By this power of changing the colour of the exposed portion of the body so as to harmonise with the surroundings, the fish is enabled to become in a large measure invisible, and in proportion to the effectiveness of the change to escape its enemies. Some soles are quite invisible.
