no THE ANATOMY OF VERTEBRATED ANIMALS. 



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

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

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



PiQ . 81. — Side and upper tIcwb of the brain of Petromyzon Jtv/daUUe, and an upper and 

 inner view of tlie membranous labyrinth of P. Tnarinus. The following letters refer to 

 the flgnres of the brain : I., the olfactory nerves, narrow anterior prolongations of the 

 rhinencephalon (A) ; B, the prosencephalon ; C, the thalamencephalon ; D, 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 ; lY., 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, 1', 2, 2^, sensory and motor roots of the first two spinal nerves. In the figure 

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

 circular canals, which correspond mtb the anterior and posterior vertical canals of other 

 Ynftebrata ; iZ, their union and conamon opening into the vestibule ; 6, the ampullro. 



blindly belovr and behind, but in the Hags (Myxine), it opens 

 >nto the pharynx. In no other fishes, except Ziepidosiren^ does 

 the olfactory apparatus communicate with the cavity of the 

 mouth. 



The reproductive organs of the llarsipohranchii are solid 

 plates suspended beneath the spinal column, and Ihey have no 



