the Position of the Eyes in Pleuronectidas. 381 



instructive treatise " 06 the Asymmetrj of the Pleuronectidge " 

 (Trans. Linn. Soc. 1866, vol, xxv. p. 263)*. The fact, however, 

 that the young fish, at least those examined bj me, possess 

 the same number of rays in the dorsal fin as the adults, would 

 in any case render this theory very doubtful, even if it had 

 not now been proved by the observations above detailed 

 that the eye of the Mind side does not only glide over from 

 its own to the eye-side of the fish^ hut^ when arrived here^ it 

 recedes a little along the dorsal fin. It is consequently the 

 eye which moves round the anterior end of the fin, not the 

 fin that prolongs itself past the eye. A comparison between 

 different specimens of the same species shows that the change 

 of place is rather a slow process. On the head of the young 

 Rh. harhatus above referred to, which is 18 millims. long, the 

 eye is still quite in front of the fin ; on another specimen 

 of the same species which I have before me, and which 



species 



is 



45 millims. long, the centre of the eye is on a level with 

 the second and third ray of the fin ; on a third specimen, 

 90 millims. long, the eye is on a level with the third and 

 fourth rays, and on a fourth specimen, 115 millims. long, it 

 has receded as far as the fourth and fifth rays. 



The total length of the first-described young P. platessa was, 

 as stated, 10 millims. including the caudal fin. If, then, we take 

 into consideration the proportionally great size of the eggs of 

 this species, which measure 2 millims. in diameter, as well as 

 the fact that the fish-embryo is, as it were, rolled up inside 

 the ovum, it becomes clear that the specimen in question must 

 have been caught not many days after having left the ^^^. 

 Even allowing a margin for more rapid growth during the 

 first days after hatching, it must be conceded, on comparing 

 the three specimens above described, respectively 10 millims., 



* Dr. Traquair ascribes to Fr. Rosenthal the opinion that the upper 

 eye of the flounders attains to its anomalous flnal position by passing 

 under the dorsal fin right through the head ; but, although Rosenthal's 

 expressions (Ichthyotomische Tafeln, ii. 3, Berlin, 1821, 4. p. 5) are vague 

 enough, and may be so interpreted, I think that he may equally well be 

 supposed to have intended only, by a sort of figure of speech, to illustrate 

 the peculiarity of the flounder-skull. The idea that a highly complicated 

 organ, after having attained its full development, should loosen itself 

 from the ground out of which it has grown, wander about amongst other 

 utterly different organs of the body — nay, even go right through the 

 body of the animal, in order to turn up again on the other side and 

 take root there — this idea is one which scientific zool-^gy now-a-days can 

 only put aside as a curiosity. I therefore think it due to the memory of 

 so able an anatomist as Rosenthal that such a crude opinion should not 

 be ascribed to him except on more cogent grounds, although the plate 

 in the explanation of which the expressions in question occur was pub- 

 lished as early as 1821 — that is, only a few years after the foundation of 

 comparative anatomy by Bichat. 



