Chap, xii.] OLFACTORY ORGANS. 441 



pit-shaped cavity ; the epithelial cells that line this 

 pit are the end organs of the olfactory sense, and the 

 whole layer forms the so-called Sdmeideriaii 

 membrane, which gradually becomes more and 

 more elaborately folded. The sac does not remain 

 pit-like in fishes, but becomes connected by a groove 

 with the angle of the mouth ; this groove, which may 

 become of some depth (rays), is covered over by a fold 

 of the integument, the so-called nasal valve (Fig. 188) ; 

 so that we are able to distin- 

 guish an anterior and a pos- 

 terior orifice, the hinder of 

 which is in close relation to, 

 but is not within, the cavity of 

 the mouth. 



Tn the Dipnoi the hinder 

 orifices come to lie within the 



buccal area, and the same is Fig m _ Nasal Groove 

 true or all the pentadactyle V er- of the Dog-fish, 



tebrata, in which, as we ascend * 1 Sg^ffyUfS\ r\ 

 the series, we find the posterior S!f roove - (AfterGegen- 

 nares coming to lie farther and 



farther back, as the various bones of the roof of the 

 mouth form outgrowths which serve as a floor for the 

 nasal passages. We cannot resist the supposition that 

 this movement in the position of the posterior nares 

 is in relation, firstly, to the altered mode of respiration, 

 the lungs taking the place of the gills ; and, secondly, 

 to the needs of the organism. If we may judge from 

 the crocodile or the whale (page 242), the elongated 

 passage has not essentially any relation to the olfactory 

 sense ; the true olfactory portion remains throughout 

 the Vertebrata a closed pit, and the only advantage to 

 it that results from the elongation of the passage is a 

 mechanical one. The longer air passages allow of a 

 more forcible inspiration, and, in consequence, of a 

 more forcible taking in of odoriferous particles. 



