298 SCOTT. [Vol. I. 



lamprey is derived from the same primitive type as that of the 

 higher vertebrates ; the type shown in the early stages, which, as 

 already indicated, is the ordinary one. But whether in the dif- 

 ferentiation of the ear in the lamprey it has passed through 

 stages higher than the present condition, and more like that of 

 the typical fishes, cannot from present evidence be decided. 



The ear is the only one of the higher sense-organs which is 

 not greatly retarded in its larval development. No such great 

 advance occurs at the time of metamorphosis as takes place in 

 the olfactory organ and eye. The reason of this difference is 

 probably to be found in the fact that the possession of an organ 

 of hearing is of value even to the larva buried in mud, while 

 those of smell and sight could hardly be employed at all under 

 such conditions. 



Regarded as a whole, the sense-organs of Petromyzon do not 

 show degeneracy, but rather a retardation of development. There 

 are also certain minor peculiarities which appear to have been 

 acquired within the Cyclostomatous phylum, such as the union 

 of the nasal pits into a median unpaired organ and the develop- 

 ment of the naso-palatal canal ; the peculiar structure of the 

 retina, as described by Langerhans and Miiller; the absence of 

 the horizontal semicircular canal ; the division of the vestibule 

 into chambers, and the presence of the auditory appendix. 

 These characteristics are remarkable ; but they cannot fairly be 

 called degenerate, or, at least, there is no sufficient evidence for 

 so regarding them. 



SUMMARY. 



(i.) The upper lip rotates through an arc of i8o°, and this 

 rotation has a great effect upon the development of the anterior 

 organs of the head. 



(2.) All parts of the brain, except the olfactory lobes, are 

 present in the freshly-hatched larva. The brain is exceed- 

 ingly small, particularly the fore and mid-brains, due, no doubt, 

 to the undeveloped condition of the sense-organs during most 

 of larval life. Even the hind-brain, though large in proportion to 

 the anterior divisions, is absolutely small, 



(3.) The cranial flexure is always slight, and is partially 

 corrected by a rotation in the opposite sense. 



(4.) The hemispheres arise as an unpaired solid mass, which 



