550 ME. H. M. KYLE ON NASAL 



would be most in action, opening and alintting synchronously 

 with the gill-covers. "When the gill-covers rise, and so increase 

 the cavity of the mouth, whilst the posterior edges of the 

 branchiostegal membranes close the gill-openings, this fold 

 will also rise, and water will enter the mouth from the nasal 

 passages. Conversely, when the gill-covers fall, the fold will 

 press on the internal nares and close them ; whilst the water 

 from the mouth, passing between the gill-arches, bathing the 

 gills, will escape by the gill-openings *. 



The foregoing discussion of the functions of the nasal sacs and 

 their specialization, leads on to a consideration of Huxley's 

 conclusions with respect to thq use of the communication between 

 the nose and mouth in the fishes with which he dealt. In his 

 paper on Ceratodus (1. c), after comparing the Dipnoi and Selachii 

 with regard to the nasal organ, he raised the question — of what 

 use are such nasal passages and internal nares to purely bran- 

 chiate animals ? In answering this, he considered that in all 

 probability they are primarily connected with respiration when 

 the mouth is closed ; and, secondarily, that by their means a 

 constant stream of water containing odoriferous particles would 

 be brought into contact with the sensory epithelium of the 

 olfactory organs. L 



What has been advanced in the foregoing pages is so far in 

 complete accord with both of these conclusions, but Huxley went 

 beyond this and, reasoning from the second, concluded that the 

 posterior nostrils of the Teleostei, where they occur, have most 

 probably a function similar to the internal nares, viz., to aid in 

 ensuring the adequate passage of odoriferous particles over the 

 sensory epithelium. 



This conclusion is, however, open to doubt. When internal 

 nares are present, the pumping action which draws the water 

 through the nasal passages is carried on by means of the gill- 

 covers and floor of the mouth iu common with that concerned in 

 respiration. Eut in those Teleostei where internal nares and 

 nasal sacs are absent and posterior nostrils are present, this 

 pumping action during respiration cannot involve the olfactory 

 organ. Hence the essential conditions which would render the 



* Howes has proposed to distinguish this buccal mechanism of respiration 

 characteristic of all the Ichthyopsida as stomatophysous, and that characteristic 

 of the Amniota, and taking place only in the presence of a costal sternum, as 

 somatophysotis. Cf. Jour. Anat. & Phys. vol. xxiii. p. 272. 



