THE BREATHING APPARATUS, ETC., OF FISH 31 



family, the nostrils bear a similar relative position to the mouth 

 as in the higher vertebrates ; they are covered externally 

 with skin pierced by two openings, the foremost of which 

 is fitted with a valve, the posterior being usually open. Each 

 nostril leads into a separate sac with a membranous lining, 

 in which the olfactory nerve spreads its ramifications. The 

 exception to this general arrangement is a remarkable one, 

 furnished by the lamprey and lampern, which have but a single 

 nostril situated on the top of the head, slightly in front of the 

 eyes. 



