NOSTRILS 227 



growths, supplied from the aortic arches, might give rise to the 

 bladder of Polyptems; while from their more complete fusion 

 dorsally might be formed the bladder of Amia, etc. The return of 

 the blood to the veins behind the heart is difficult to explain on 

 this hypothesis, and it must be confessed that the problem of the 

 homology of the air-bladder is not yet satisfactorily solved. 



The primitive position of the nostrils seems to have been in the 

 Osteichthyes, as in the Chondrichthyes, on the ventral surface of 

 the snout. Such it was apparently in the early Teleostomes (Osteo- 

 lepidae), and such it remains in the Dipnoi. But in all the living 

 Osteichthyes, with the exception of a few specialised genera (p. 445), 

 the nostrils are double. The groove on either side of the fronto- 

 nasal process, already described in the Elasmobranchs (p. 125), 

 closes over to form a complete canal in the Dipnoi ; so that in the 

 adult the nasal sac communicates with the exterior by an external 

 anterior nostril, and with the buccal cavity by an internal posterior 

 nostril, as in Pentadactyle vertebrates (Fig. 207). In the Acti- 

 nopterygii and Polypteridae the nostrils remain separated from the 

 mouth, and migrate towards the dorsal surface of the snout. No 

 distinct fronto-nasal process, and no groove to the mouth develop ; 

 but the nostril becomes subdivided into two by a narrow bridge. 

 The two openings so formed are probably homologous with the 

 internal and external nostrils of the Dipnoi. There are no movable 

 eyelids. 



The Osteichthyes, with the exception of the specialised Teleostei, 

 retain many primitive characters such as the spiral valve in the 

 intestine, the contractile conus of the heart with many rows of 

 valves, and the chiasma of the optic nerves. The urinogenital 

 organs, also, are built on much the same plan in the primitive 

 forms as in the Elasmobranchs ; but the cloaca is lost in all living 

 forms except the Dipnoi. 



We conclude that the development of true bone in the 

 endoskeleton, of scales on the body, of lepidotrichia on the fins, of a 

 special set of bony plates on the head and shoulder-girdle, of marginal 

 jaw-bones, of a connection between the dermal shoulder- girdle and 

 the skull, of pleural ribs, of an operculum covering the gill-openings, 

 of an air-bladder, and of double nostrils, justify the inclusion of 

 the Dipnoi and Teleostomi in a separate sub-grade the Osteichthyes. 



