PLEURONECTID^E. 3 



that when the young fish is seen in profile, the eyes of the two sides no longer 

 appear in the same place, that on the blind side being now slightly above and in 

 advance of that on the coloured side. With increasing age the eye on the blind 

 side rises higher and higher towards the median longitudinal line of the head. 

 The dorsal fin gradually extends towards the nostrils, and finds its way behind 

 the eye which has come from the blind side. The eye is transferred at such an 

 early period that the bones of the skull are cartilaginous, and the transfer is 

 carried out by a combined process of translation and rotation. In some cases it 

 was observed to be transferred, as described by Malm, round the head by the 

 snout, and in others to actually pass through the soft tissues of the head, and 

 this divergence appeared to be due solely to the generic differences in the position 

 of the dorsal fin. 



Some flat-fishes have the eyes normally on the right side of the body, others 

 on the left, but reversed instances, or those in which the coloured side is on that 

 which is as a rule uncoloured, are not uncommon among most genera of 

 Pleuronectoids. Such is very frequently observed in flounders, these fishes, living 

 close in shore, being* more exposed to the actions of currents than those genera 

 which live in the deep sea. In this way they become in their very early life forced 

 on to the side which is not the normal one. Such valuations are more commonly 

 perceived in some localities than in others, and will generally be found due to local 

 disturbing causes. Professor Agassiz kept young flounders in glass vases, raised 

 high above the table, and found that, notwithstanding the fact that here no 

 disturbance could occur, seven out of fifteen were noticed endeavouring to force 

 the eye round the wrong way, by lying down on the opposite side. But all the 

 seven soon died, and this may account for the comparative scarcity of reversed flat- 

 fishes. He also observed that the presence of light on all sides failed to arrest, or 

 even to delay, the transfer of the eye, and the consequent change in the colour 

 of the under side, which invariably became white with advancing age, when that 

 organ passed over to the upper surface. Thus it would appear that the absence of 

 light is not the primary or sole cause of the want of colour on the under side of 

 the flat-fishes. 



While the eye is progressing from the one side of the head to the opposite where 

 it is to find its final resting place, its course is occasionally arrested, and it may 

 remain permanently stationary on the upper surface of the head, and this is 

 commonly seen in what is known as " double flat-fishes,"* or those coloured on both 

 sides. f Donovan obtained a young double turbot, and thinking he had discovered 

 an unknown fish, termed it Pleuronectes cyclops, and which he figured on plate xc. 

 " So singularly different," says that author, " is this from the rest of the 

 Pleuronectes that it seems to militate even against the chai'acter of the genus, 

 which requires that both eyes should be placed on one side. We were almost 

 tempted, from this consideration, to constitute a new genus for this curious fish." 

 Yarrell remarks, " The Pleuronectes cyclops of Mr. Donovan, I believe to be an 

 example of the young fry of the turbot, the head of which is not perfectly formed." 

 Double fishes have been observed in flounders, turbot, plaice, soles, etc., and they 

 are seen to swim vertically, and to be more frequently found near the surface of 

 the water than those which progress in a normal manner. All who have eaten 

 these double flat-fishes know how they are most deservedly held in greater estima- 

 tion for the table than others which have an uncoloured as well as a coloured side. 

 Then there are albinos or those which are uncoloured or nearly white on both 

 sides (being almost or quite double albinos) and still retaining their normal form ; 

 but in some that have been carefully examined no sexual parts could be detected. 



* These would appear to be examples in which the ancestral form has been more strongly 

 developed than has the adopted. The early lives of these fishes afford excellent illustrations of these 

 two distinct phenomena, or of ancestral or inherited form and adopted modification. For in the 

 earlier stages of their existence after leaving the egg, they swim on edge with an eye on either side 

 of the head as in other fishes; but in a definite period they assume their adopted form of both eyes 

 being on the same side. No matter through how many countless generations these pleuronectoids 

 may pass, still the embryonic or early fry condition, or an eye on either side of the head, will continue 

 to be apparent, just as the visceral clefts in the human embryo are. 



t In many double flat-fishes the eves are in their normal position. 



1* 



