xix] 



SENSE-ORGANS 



419 



which usually have at the base several root-like branches. The 

 front end of the brain comes in direct contact with the wall of the 

 nasal sac and the axons of the sensory cells stretch into the brain, 

 thus constituting the olfactory nerve (Fig. 207). 



10 



"IG. 209. Ear of Chimaera inoiistiosa L. x about 4. From Retzius. Seen 

 from the inner side. 



External aperture on roof of skull. The wall of 2, the " ductus endo- 



lymphaticus," is partly removed to show that it is a tube. 3. Anterior, 



4, posterior, and 5, horizontal semicircular canals. 6. Anterior, 



7, external, and 8, posterior ampullae. 9. Sacculus. 10. Auditory 

 or 8th nerve. 



The ears are also at first pits of the skin placed further back at 

 the sides of the hind-brain. In the lower forms these pits retain a 



272 



