FLA T-FfSHES. 545 



animal's; movements. It is also worthy of note that those fresh-water 

 fishes (Ostariophysise) which have the adjusting mechanism above re- 

 ferred to, have a marked ascendancy over all other fresh-water species 

 in which this mechanism is awanting (Bridge and Haddon). 



Fiat-Fishes. In illustration of biological problems, let us briefly 

 discuss some of the peculiarities of the flat-fishes (Pleuronectidre), such 

 as flounder, plaice, sole, and turbot. These forms, we at once perceive, 

 are flattened from side to side, unlike 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 unpigmeiited. 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 (argen- 

 teum) and minute reflecting elements (iridocytes), but no pigment cells 

 (chromatophores) ; all three contribute to the colour of the upturned 

 surface. 



In early life the larvre 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 P/agiisi'a it passes 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 disappear ; 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 sometimes 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. 



So far some of the more important facts, -what of their interpreta- 

 tion? That these asymmetrical forms have been derived from sym- 

 metrical ancestors is plainly suggested by their development. Of the 

 original cause of the asymmetry we are quite ignorant. Did changes in 

 the conditions of life induce the ancestral forms to leave the surface for 

 the bottom ? Or was the change due to certain peculiarities of structure, 

 requiring, of course, previous explanation, such as the great depth of 

 the body and the degeneration of the swim-bladder? Or did both 

 these causes operate at once ? 



But supposing we had attained to some clearness in regard to the 

 change of habitat and loss of vertical balance, we should then have to 



