174 FISH GALLERY. 



Flat- The fishes of the family Pleuronectidse, commonly known as 



s es ' Flat-fishes (figs. 83 and 84), lie upon their side when resting on 

 the hottom of the sea. The front part of the skull is twisted 

 in such a manner that the two eyes are on the upper surface, 

 and as may be seen by reference to the cranium of the Halibut, 

 755, the hinder part of the skull that contains the brain does 

 not share in the torsion. The under surface of the fish, known 

 as the t( blind " side, is flat and white in colour, whereas the 

 upper surface is pigmented and more convex. The dorsal and 

 anal fins are extensive and form a kind of fringe to the flattened 

 body. The paired fins are often reduced, sometimes absent. 

 There is no swim-bladder. The Pleuronectidse constitute a large 

 family of fishes, almost all of them marine. They are valuable 

 food-fishes and are represented in British waters by the Sole 

 Turbot, Brill, Plaice, Halibut, &c. In the Turbot and Brill the 

 eyes are on the left side, in the others they are on the right side. 

 The food of the Flat-fishes consists principally of molluscs, 

 crustaceans and sea-worms, but the Turbot and Brill feed on 

 fishes, such as the Launce, Herring, Whiting, and even small 

 fishes of their own family, such as the Sole and Plaice. 

 Adalah. The least specialised of the family Pleuronectidse is the Adalah, 

 Psettodes erumei, 752 and 753, fig. 83 A, of the Indian Ocean and 

 seas of China and Australia ; the pelvic bones and fins are placed 

 as in an ordinary fish such as the Perch, and the dorsal fin does 

 not extend on to the head. It has been pointed out above that 

 the two eyes of the Flat-fishes are on the same side of the body, 

 but this does not apply to the very young. The fishes commence 

 their existence as perfectly symmetrical fishes with their eyes 

 on opposite sides of the head and their jaws similar on the right 

 and left sides. As development proceeds and the body becomes 

 more and more flattened, one of the eyes moves to the edge and 

 then over to the same side of the body as the other eye, owing 

 to a twisting of the front part of the skull. The Adalah is 

 interesting in that it retains more completely the bilateral 

 symmetry of the young than do any of the other Flat-fishes. 

 The eye which migrates — sometimes the right, sometimes the 

 left, since both right-handed and left-handed forms occur in 

 this species (compare 752 and 753) — remains on the edge of 

 the head, and does not come over completely on to the pigmented 



