182 THE COMMERCIAL SEA FISHES OF 



The flat fishes are among the most remarkable of verte- 

 brate animals, as for about a week or more after birth they 

 swim on edge in a manner similar to other fishes, having their 

 back fin above, their anal fin below them, and possessing 

 an eye on either side of the head. But as they grow older 

 this erect position becomes lost, their sides become their 

 upper and lower surfaces, while both eyes are on the 

 superior or coloured side of the body. The adult, when 

 at rest or swimming, usually keeps near the bottom of the 

 water, and progresses by means of a sort of undulating 

 motion of the whole body, and of the unpaired fins. These 

 fish are of a broad, flat shape, and margined in almost their 

 entire extent by the dorsal, caudal, and anal fins ; while 

 not only the muscles, but the skin, the gills, gill-covers, and 

 even the pectoral fin-rays, are less developed on the blind 

 (or normally under surface) than on the coloured side, the 

 mouth also being, as it were, bent round to this eyeless 

 side, towards which the anterior part of the face seems to 

 be twisted. From a very early age it had been known that 

 these fishes when first emerging from the ova, and while in 

 a pellucid condition, have an eye on either side of the 

 head ; that by degrees the eye, on what eventually will be 

 the eyeless side, becomes depressed, while at the same time 

 a dark spot appears on the opposite side of the head, so 

 that the fish almost seems to possess three eyes. By 

 degrees this dark spot becomes a distinct eye, while that 

 on the other side gradually disappears ; in short, the eye 

 apparently migrates from what is henceforth known as the 

 blind side of the fish. Professor Alexander Agassiz re- 

 marked that the first change and the process is identical, 

 whether we take a right or a left flounder. First there is 

 a slight advance towards the snout of the eye about to be 

 transferred, so that the transverse axis, passing through the 



