FLAT-FISH(Pleuronectidae) is the name common to all those fishes which swim on their side, as the holibut, turbot, brill, plaice, flounder, sole, &c. The side which is turned towards the bottom, and in some kinds is the right in others the left, is generally colourless, and called “blind,” from the absence of an eye on this side. The opposite side, which is turned upwards and towards the light, is variously, and in some tropical species even vividly, coloured, both eyes being placed on this side of the head. All the bones and muscles of the upper side are more strongly developed than on the lower; but it is noteworthy that these fishes when hatched, and for a short time afterwards, are symmetrical like other fishes, swimming with the body in a vertical position, and having the eyes placed normally, viz., one on the right and the other on the left side. With advancing growth one of the eyes is gradually pushed to the opposite side, and the fish assumes its horizontal position in the water. Flat-fishes are bottom-fish, rarely rising to the surface, and apparently not descending to very great depths. They are found in all seas, except perhaps the highest latitudes. More than 200 species are known; they are most numerous towards the equator, whilst those of the largest size (holibut) occur in the temperate and cold regions. Some enter fresh water freely, like the flounder. The size and abundance of the flat-fish, and the flavour of their flesh, render this family one of the most useful and economically important. See Ichthyology.