FLATFISH is the name of the fishes of the order Heteroso mata, which differ from all other fishes in having both eyes on one side. They live at the bottom, with the eyed side uppermost; this is coloured, whereas the blind or underside is normally white; but exposure of this side to light will cause some development of pig ment. Nearly all flat-fishes are marine, and they have minute floating eggs; the larvae are symmetrical, with an eye on each side, and swim near the surface of the sea; but after a time one eye moves round over the top of the head to the other side and the little fish, falling over gradually on its side, sinks to the bottom. In all but the primitive Psettodes the dorsal fin grows forward on to the head and in some with a long larval life this happens before the eye migrates, so that in pushing its way between the fin and the head it seems to go through the head from one side to the other. Many flat fishes are remarkable for their power of chang ing their coloration in order to resemble the ground on which they lie, and it has been established by experiment that they must see the ground in order to look like it. The eyes do not look upwards, but in life stand out from the head and can be turned independ ently in different directions looking sideways, or sometimes one forward and the other backward.
Psettodes.—The most primitive genus, with two species, from West Africa and the Indo-Pacific, is typically perch-like in struc ture, and indicates the derivation of flat-fishes from some pescoid ancestor allied to the Senanidae or sea-perches. The other flat fishes, numbering about 600 species, are either flounders, with the lower jaw prominent, or soles, in which the rounded snout pro jects beyond the small curved mouth; each of these groups in cludes a family with the eyes on the right side, and another with the eyes on the left. The Pleuronectidae, or dextral flounders, include the halibut, plaice and other important food-fishes of northern seas. The Bothidae, or sinistral flounders, include the turbot ; some of the American species of Paralichthys are also val ued as food. The Soleidae, or true soles, with eyes on the right side include the well known common sole of Europe. The Cynoglo nidae, or tongue-soles, with eyes on the left side, inhabit tropical and subtropical waters.
In the more specialized forms, which feed exclusively on bot tom-living invertebrates, the jaws of the blind side are more strongly developed than those of the eyed side, and have stronger teeth ; in the soles those of the eyed side are toothless.