CHAPTER XXVII
SUBORDER HETEROSOMATA
The Flatfishes.—Perhaps the most remarkable offshoot from the order of spiny-rayed fishes is the great group of flounders and soles, called by Bonaparte Heterosomata (ἔτερός, differing; σῶμα, body). The essential character of this group is found in the twisting of the anterior part of the cranium, an arrangement which brings both eyes on the same side of the head. This is accompanied by a great compression of the body, as a result of which the flounders swim horizontally or lie flat on the sand. On the side which is uppermost both eyes are placed, this side being colored, brown or gray or mottled. The lower side is usually plain white. In certain genera the right side is uppermost, in others the left. In a very few, confined to the coast of California, the eyes are on the right or left side indifferently.
The process of the twisting of the head has been already described (see p. 174, Vol. I). The very young have the body translucent and symmetrical, standing upright in the water. Soon the tendency to rest on the bottom sets in, the body leans to left or right, and the lower eye gradually traverses the front of the head to the other side. This movement is best seen in the species of Platophrys, in which the final arrangement of the eyes is a highly specialized one.
In some or all of the soles it is perhaps true that the eye turns over and pierces the cranium instead of passing across it. This opinion needs verification, and the process should be studied in detail in as many species as possible. The present writer has seen it in species of Platophrys only, the same genus in which it was carefully studied by Dr. Carlo F. Emery of Bologna. In the halibut, and in the more primitive flounders generally, the process takes place at an earlier stage than in Platophrys.
Optic Nerves of Flounders.—In the Bulletin of the Museum of Comparative Zoology (Vol. XL, No. 5) Professor George H. Parker discusses the relations of the optic nerves in the group of flounders or flatfishes.
In the bony fishes the optic nerves pass to the optic lobes of the brain, the one passing to the lobes of the opposite side simply lying over the other, without intermingling of fibers, such as takes place in the higher vertebrates and in the more primitive fishes.
According to Parker's observations, in ordinary bony fishes the right nerve may be indifferently above or below the other. In 1000 specimens of ten common species, 486 have the left nerve uppermost and 514 the right nerve. In most individual species the numbers are practically equal. Thus, in the haddock, 48 have the left nerve uppermost and 52 the right nerve.
In the unsymmetrical teleosts or flounders, and soles, this condition no longer obtains. In those species of flounder with the eyes on the right side 236 individuals, representing sixteen species, had the left nerve uppermost in all cases.
Of flounders with the eyes on the left side, 131 individuals, representing nine species, all have the right nerve uppermost.