176 



Instincts, Habits, and Adaptations 



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- 

 nectida 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 



FIG. 130. Larval Flounder, Pseudopteuronectes americanus. (After S. R. Williams.) 



FIG. 131. 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. 



