110 



THE ANATOMY OF YERTEBRATED ANIMALS. 



a sac placed in the middle line of the head, and having a sin- 

 gle, median, external aperture. In all other Vertehrata there 

 are two nasal sacs. In the Lampreys, the nasal sac terminates 



Fis. 81. — Side and upper views of the brain of Petromyson flnmiatilU, and an upper and 

 inner view of the membranous labyrinth of P. marinus. The following letters refer to 

 the figures of the brain : I., the olfactory nerves, narrow anterior prolongations of th« 

 rhinencephalon (A); B. the prosencephalon; C, the thalamencei>halon ; I), the mesen- 

 cephalon ; E, the medulla oblongata ; F, the fourth ventricle ; e, the narrow band which 

 is all that represents the cerebellum ; G, the spinal cord ; II., the optic ; III., the oculo- 

 motorius ; IV., the patheticus ; V., the trigeminal ; VI., the abducens ; VII., the facial, and 

 the auditory; VIII., the glosso-pharyngeal and pneumogastric ; IX., the hypoglossal 

 nerves ; 1, i', 2, 2', sensory and motor roots of the first two spinal nerves. In" the figure 

 of the membranous labyrinth : k, the auditory nerve ; a, the vestibule ; c, the two semi- 

 circular canals, which coiTCspond with the anterior and posterior vertical canals of other 

 Vertebrata ; d, their union and common opening into the vestibule ; &, the ampullse. 



blindly below and behind, but in the Hags {JSTyxine)^ it opens 

 into the pharynx. In no other fishes, except Lepidoslren^ does 

 the olfactory apjDaratus communicate with the cavity of the 

 mouth. 



The reproductive organs of the Marsipohrancliii are solid 

 plates suspended beneath the spinal column, and they have no 



