24 PROCEEDINGS OF THE AMERICAN ACADEMY 



passing through the transverse axis. This specimen is perfectly 

 transparent, — fully as transparent as the most delicate Hydroid 

 Medusa. The action of the heart, the course of the vessels, can be 

 readily followed, as well as the other structural details, which are 

 usually only visible after dissection. The dorsal fin projects far 

 down the frontal ridge to the nostrils, well in advance of the eyes. 

 Fig. 2. Young Plagusia (fig. 1). Seen with head on. 



„ 3. Shows the relative position of the eyes after the first movements of 

 translation and of rotation have become visible by the slight advance 

 and rising of the eye of the right side. Seen from the left side. 



„ 4. Somewhat more advanced than fig. 2. Seen from the right side. The 

 outline of left eye can be traced through the tissues of the head. 



„ 5. Head, seen from the left side. The right eye has moved upwards suffi- 

 ciently to be seen through the tissues of the head, clear above the 

 left eye. We find in this stage the first trace of the opening of 

 the eye on the left. The eye, when turned in the socket, can look 

 through the tissues at the base of the dorsal; and, when thus 

 turned, to see through the left, is nearly as sensitive to approach- 

 ing objects as the left eye. When looking at the same fish for the 

 other side (the right), we find that the eye has deeply sunk in the 

 tissues between the frontal bone and base of dorsal fin, and that, 

 while sinking and pushing its way to the opposite side, the tissues 

 of the right side have gradually united and narrowed the former 

 large circular orbit to a mere small elliptical opening. 



„ 6. The eye of the right side, as turned to the right ; the new orbit ap- 

 pearing on the upper edge of the eyeball. 



„ 7. The same eye with the ball turned toward the left, showing the com- 

 mencement of the new orbit forming as a small circular opening 

 on the left side of the fish. The old orbit of the right side being 

 now reduced to a minimum, the fish now having two orbits on the 

 left side and one on the right. The orbit of the right being re- 

 duced to a small aperture, and disappearing in a subsequent stage 

 (fig. 9), while the new orbit of the right eye on the left side is as 

 yet much smaller than the corresponding orbit of the left eye. 



„ 8. Head seen from the right side, showing the small size of the old orbit 

 of the right eye after it has forced its way partly across the head. 



„ 9. The right eye has now passed entirely round the frontal bone, and is 

 held in its hollow curve, and has at same time forced its way through 

 the tissues so far that the original orbit of the right side has 

 become closed, and the new orbit for the right eye on the left side 

 has become nearly as large as the orbit of the left eye. 



„ 10. In this stage, the eye from the right side is now completely trans- 

 ferred to the left, and no difference is apparent between the orbits. 

 In this and all preceding stages, the great length of the two optic 

 nerves is readily seen ; and we thus understand the possibility of so 

 extensive a movement of either eye without interfering with the 

 visual function. The slack of the optic nerves being only taken 

 in for the eye which happens to be transferred in any genus of 

 Flounder. 



