Page:Natural History, Fishes.djvu/274

260 than that of being greatly depressed or flattened horizontally, like the Skates, which at first sight they much resemble: but, in fact, the coloured surface is not the back, nor the white surface the belly, in the Flat-fish, but these are truly the two sides, right and left, so that instead of being depressed, it is compressed, or flattened vertically, like the Chætodons. The latter, however, like other fishes, swim with the back uppermost, notwithstanding their thinness; but the Turbot or Sole, swims or grovels along the bottom, upon its side, the coloured side, right or left, being uppermost. The term Pleuronectes, compounded of two Greek words, signifying side-swimmer, expresses this peculiarity.

But this is not all. If the eyes had been placed like those of the Chætodon, for example, one on each side, that which belonged to the white or inferior side, would be rendered useless, since it would be almost perpetually buried in the mud of the bottom. Hence, by an unprecedented exception to the symmetry which marks the organs of sense in all other vertebrate animals, both of the eyes are placed on the same side of the head, one above the other. They are, however, frequently not in the same line, and one is often smaller and less developed than the other.

In addition to the above peculiarities, we may mention that the spine makes a sudden twist near the head to one side; that the bones of the head are not symmetrical; that the two sides of the mouth are unequal; that the pectoral and ventral fins of the under side are generally smaller than those of the upper; and that the dorsal and anal generally correspond to each other, the one fringing