THE BREATHING APPARATUS, ETC., OF FISH 3x 
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. 
