PASSAGE OF EYE IN FLAT-FISH. 



75 



and only after this rotation, the dorsal fin grows forwards beyond the 

 level of the eyes. In the second mode which is undergone by Plagmia, 

 the dorsal fin grows forwards to the snout, while the e3'es are still in 

 their initial positions. When the right eye in the coarse of rotation 

 approaches the base of the dorsal fin, it graduali}' sinks into the tissues of 

 the base of the dorsal fin, between it and the frontal ; so that the right eye 

 is now not to be seen on the surface of either side. A very interesting 

 circumstance was observed by Agassiz, that a fresh orbital opening is 

 formed for the right eye on the left side, and that the original orbit of 

 the right atrophies after the rotation of the eye ; therefore in a certain 

 stage of metamorphosis, there are three orbital openings : one on the 

 left side, the original orbit of the left eye ; a small one on the left side, 

 the new orbit for the right eye ; and a small orbit on the right side, the 

 remnant of the original orbit of the right eye. He says, " With the 

 continued sinking of the right eye, the gradual resorption of the tissues, 

 and the closing up of the old orbit, as the eye works its way across the 

 head, we eventually get the right eye entirely over to the left side. It 

 has now, by a movement of translation and of rotation, penetrated 

 through the tissues between the base of the dorsal fin and the frontal 

 bone ; having apparently passed through the head, as was suggested to 

 Steenstrup, by his examination of the alcoholic specimens which 

 furnished him the materials for his paper on Playusia." 



In the present flat-fish, the dorsal fin also grows forward before the 

 rotation of the right eye, but this anterior extension does not unite 

 with the head, and there is a distinct hole bounded by the head and the 

 anterior extension of the dorsal fin, for the passsge of the right eye, 

 which travels round the dorsal side of the head without sinking into its 

 tissues. The orbit of the right eye travels also with the rotation of the 

 head from one side to the other, as in the case of a number of fiat- 

 fishes. Now if we compare this mode of the passage of the eye with 

 the first mode described by Agassiz, which is undergone by a majority 

 of flat-fishes, we find little difference between them, beyond the fact 

 that before the rotation of the eye takes place, the dorsal fin grows in 



