Instincts, Habits, and Adaptations Ty, 
12. The only well-marked asymmetry in the adult brain is 
due to the much larger size of the olfactory nerve and lobe of 
the ocular side. 
13. There is a perfect chiasma. 
14. The optic nerve of the migrating eye is always anterior 
to that of the other eye. 
“The why of the peculiar metamorphosis of the Pleuro- 
nectide is an unsolved problem. The presence or absence of 
a swim-bladder can have nothing to do with the change of 
habit of the young flatfish, for P. americanus must lose its air- 
bladder before metamorphosis begins, since sections showed no 
Fie. 61.—Larval Flounder, Pseudopleuronectes americanus. (After S.R. Williams.) 
evidence of it, whereas in Lophopsetta maculata, ‘the window- 
pane flounder,’ the air-sac can often be seen by the naked eye 
up to the time when the fish assumes the adult coloration, and 
long after it has assumed the adult form. 
“Cunningham has suggested that the weight of the fish 
acting upon the lower eye after the turning would press it 
toward the upper side out of the way. But in all probability the 
planktonic larva rests on the sea-bottom little if at all before 
metamorphosing. Those taken by Mr. Williams into the labora- 
tory showed in resting no preference for either side until the eye 
was near the midline. 
