FLATFISH, a fish of the Heteromomata, including the families Pleuronectidee and So leida, which are characterized by their greatly compressed form, distorted skulls and the habit of lying and moving on the side. They are somewhat related to the cod family, and inhabit the seas of all parts of the world, more than 500 species, classified in some 55 genera, being known to ichthyologists. All are oval or ellip tical in outline, very thin and flexible, and habitually rest, half covered, or move sidewise along sandy sea-bottoms, some species near shore or even ascending rivers, others only in deep water. They are carnivorous, taking as food mainly mollusks, worms, sand-dollars and the like, found in such places, for the crushing of whose shells they are provided with strong teeth, chiefly developed on the inferior side of the mouth. The flatness of these fishes is not a broadening laterally but a compression of the body whereby they become high and thin in a dorso-ventral direction, lie over on one side (usually the left), and have the head perma nently twisted to the side uppermost in their customary position. This, however, comes about only with age. The very young flatfish are born in the normal shape of fishes, and swim in the ordinary vertical position, but soon begin to assume the adult peculiarities. This tendency causes profound changes in structure. The skull gradually becomes so distorted that its facial part is twisted at right angles to the plane of the cranial part, and both eyes, set close together, look upward, so that the fish has a seeing and a blind side. In order to get into this position the eye of the under side is ro tated around, or in some species right through the young skull, when the fry is only an inch or so in length and the bones not yet ossified. The curious condition of the dorsal fin in the flatfish is nevertheless a greater mark of dis tinction. The external ethmoid bone belonging to the blind side is much enlarged, and sends back a process outside the eye belonging to that side to meet another process from the cranial region of the skull. Thus, says Cum. ningham, the eye which has migrated is en closed in a complete bony orbit, while the other (lower) eye is merely bounded on its outer side by the jaw muscles. It is on this bony bridge entirely foreign to the anatomy of an ordinary fish that the dorsal fin is supported, and is able to extend from the tail clear to the snout, not passing between the eyes, but sepa rating the hidden from the exposed side. Simi
larly the ventral and anal fins form a continu ous growth along the ventral keel, defining there the upper side from the lower. The les ser forward fins, like the mouth, are usually asymmetrical. They are able to swim well when they please, and sometimes appear at the surface, progressing with a curious sideways undulatory movement very graceful. This is a dangerous excursion, however, and flatfish as a rule cling to 'the bottom, trusting to be over looked because of their close resemblance to the sand or mud.
The young flounder or other species is brown on both sides alike; but when it turns on one side and lies flat, the under side becomes nearly white while the upper assumes the color of the bottom upon which the fish habitually rests. This is an excellent example of protec tive resemblance (See COLORATION, PROTEC TIVE) ; and that it arises from the habits of the fish and is the effect of persistently keeping the under side in shadow, is plain from the fact, demonstrated by experiment, that when flatfish are confined in aquaria with glass bot toms, through which light is reflected upon the under side, color will develop there. Some flatfish are spotted, increasing their likeness to a pebbly bottom. All these circumstances make it evident that flatfishes originated from sym metrical ancestors related to those of the cod, but what were the inducing causes cif the extra ordinary changes that have characterized their evolution are not plain. Little help is derivable from paleontology for fossil flatfishes are ex tremely rare, the oldest known being a sole-like form from the Upper Eocene of southern Europe.
The group includes two families, the floun ders (Preuronectida) and the soles (Soleida). Jordan subdivides the former into three sec tions, namely Hippoglossiner, halibut tribe; Plewronectince, flounder tribe; and Psettina, turbot tribe. The soles are a small family dis tinguished by the hidden, adnate character of the gill-cover, the small, much twisted eyes and mouth, and the absence of teeth. The whole group yields excellent flesh, and furnishes some of the most important food-fishes of commerce as well as several regarded as especial delica cies. They are treated of at length in Goode's 'Fishery Industries,' Sec. 1 (1884) ; and in Jor dan and Evermann's 'Fishes of North and Middle America' (1898). See FLOUNDER; FLUKE; HALIBUT; PLAICE; SOLE; TONGUE-FISH; TURBOT.