34 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The optic lobe of the left 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 this 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 the developing flounder, is 
described as the recessus orbitalis by Holt (94). It is shown in cross 
section at vec. 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 from a dissection of the adult fish. The oculo- 
motor nerve (///.) supplying the transposed eye passes toward the eye- 
less side before it divides into the four customary branches. The fourth 
cranial nerve (JV.) is still more noticeably changed in its direction. In 
the cod this nerve lies near the median 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 larvee, and show that nerves retain throughout life, 
as far as possible, their phylogenetically normal 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 
