AQUATIC MAMMALS 



The remainder of the nasal diverticula is more puzzling. They occur 

 somewhat nearer the surface and may be either forward, backward or to 

 the side of the main nasal passage. Histologically the mucosa of these 

 diverticula consists of simple, stratified epithelium without speciali- 

 zation. To them has at times been assigned the function of a hydro- 

 static organ, which theory I consider to be untenable. They may orig- 

 inally have been a phylogenetic relic, like the nasal diverticula of the 

 Equidae are supposed to be, since become somewhat more complex but 

 still without useful function; they might serve in some slight measure 

 to divert water that might slop over into the open blowhole, which 

 would hardly be of sufficient importance to account for their existence. 



FIGURE 15. Diagramatic representation of the probable blow-hole action, closed 

 and open, of the porpoise Tursiops: (a) external orifice; (b) supracranial 

 part of nostril; (c) premaxillary diverticulum ; (d) bony nares; arrow in- 

 dicates direction of movement in opening, and broken line the outline of 

 the skull. 



Winge (1921) considered that "the air, which is exposed to strongly 

 varying pressure and temperature, has a tendency to provide itself with 

 greater space by widening out the nasal passage and Eustachian tube 

 wherever it meets with least resistance." For all we know this may 

 be the case and no one can prove the contrary; but I believe there has 

 been a more primary stimulus for the initial development, or at least 

 retention from a primitive ancestor, of the system of nasal diverticula 

 of the odontocetes. Let us liken the narial passage to the soft, elastic, 

 rubber tube which it so greatly resembles. The intricate and laminated 

 muscle layers which converge to operate its opening cannot function 

 as dilators with precisely equal force at each and every point, however, 



[98] 



