34 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The optic lobe of the loft side is usually cut first in cross-sections, when 

 one begins the cutting at the anterior end of the animal, as is plain from 

 the relative positions of the two in tiiis specimen (Fig. 8). The course 

 of the optic nerve to the transposed (left) eye is shown by dotted lines 

 (//. s.) in the figure. Its slack condition allows the eyes to be thrust 

 upward when the fish is buried in the mud or sand. One or two move- 

 ments of the fins will cover a fish with loose sand ; except for the pro- 

 jecting eyes, the animal is then entirely concealed. This protrusion of 

 the eyes is done by means of the so-called orbital heart. This organ, 

 mentioned by Agassiz in his description of tlie developing flounder, is 

 described as the recessus orbitalis by Holt ('94). It is shown in cross 

 section at rec. orb. in Figure 18 (Plate 4). 



A side view of the same brain as that shown in Figure 8 (Plate 2) is 

 seen in Figure 9, which makes clearer the position of the brain with 

 reference to the eyes ; but in the dissection the left eye has been raised 

 somewhat from its normal position in order to show the eye muscles 

 and the location of the optic nerves, which are purposely shaded some- 

 what darker than the surrounding muscles. 



In all the flatfishes which I have examined, the optic nerve from the 

 transposed eye is dorsal (anterior) in the chiasma. In P. americanus 

 the right optic tract and the left optic nerve are anterior (dorsal) to the 

 corresponding parts of the opposite sides (Fig. 12), whereas in Bothus 

 the left tract and the right nerve are anterior (dorsal). 



Figure 11 is drawn froni a dissection of the adult fish. The oculo- 

 motor nerve (HI.) supplying the transposed eye passes toward the eye- 

 less side before it divides into the four customary branches. The fourth 

 cranial nerve (IV.) is still more noticeably changed in its direction. In 

 the cod this nerve lies near the mediaii plane, at a distance from and 

 above the eyeball ; but in the flounder the fourth nerve of the migrat- 

 ing eye lies in contact with the eyeball and rests on the dorsal rectus 

 muscle. The optic nerve (Figs. 8, 11) also shows before reaching the 

 eyeball a bending in the same direction as that which the eye-muscle 

 nerves exhibit. These alterations in the directions of the nerves in the 

 adult indicate the nature and the place of the transposition which we 

 have followed in the larvae, and show that nerves retain throughout life, 

 as far as possible, their phylogenetically nT)rmal position. I was unable 

 to find from my dissections that the flounder, P. americanus, has a cuta- 

 neous branch of the fifth nerve. If it has, the nerve must be small. The 

 fifth has a mandibular, a maxillary and a superior ophthalmic branch. 

 The large ophthalmicus profundus of the cod is represented in the flounder 



