FLAT FISHES. 521 



such as flounder, plaice, sole, and turbot. These forms, 

 we at once perceive, are flattened from side to side, un- 

 like the skates and rays, which are flattened from above 

 downwards. 



In adult life they swim and rest on one (the right or the 

 left) side, and the hidden side is unpigmented. . Moreover, 

 the eye belonging to the downward side has come to lie 

 beside its fellow on the upward side; the dorsal fin is 

 extended anteriorly, separating the blind side of the head 

 from that which bears the eyes ; the inter-orbital parts of the 

 frontal bones, which should be median, are bent to the 

 upward side and compressed; and there may be further 

 asymmetry in the skull, as in the greater development of 

 jaws and teeth on the downward side. The skin of the 

 downward side has an opaque reflecting layer (argenteum) 

 and minute reflecting elements (iridocytes), but no pigment 

 cells (chromatophores) ; all three contribute to the colour 

 of the upturned surface. 



In early life the larvae swim for some time near the surface, 

 and in the normal position, with the dorso-ventral plane 

 vertical. Then they have an eye and chromatophores on 

 each side. As they grow older they cease to swim vertically ; 

 one eye begins to move round the edge of the head (in 

 Plagusia through an anterior extension of the dorsal fin); 

 the body is held in a slanting position so that the line join- 

 ing the eyes is kept horizontal; more or less rapidly the 

 slant increases ; the lower eye gets quite round to the 

 upward side ; the chromatophores on the shaded side dis- 

 appear ; and the fish rests and swims on one side at the 

 bottom. In the turbot the right side is normally downward; 

 in the flounder, the left side, but reversed specimens 

 (especially of flounder) often occur. Occasionally these flat 

 fishes are pigmented on both sides, and then it is some- 

 times noted that the migrating eye has not completed its 

 movement. 



Turbot and brill (species of Rhombus] have a well 

 developed swim bladder during metamorphosis, and swim 

 near the surface until the change is almost complete ; 

 flounder and other species of Pleuronectes have no swim 

 bladder during metamorphosis, and begin to lie on the 

 bottom almost as soon as the change commences. 



