RESPIRATORY ORGANS 



28 5 



all cases quite clear. In Sal mo it is said that there is no 



hyoidean hemibranch, and that the pseudobranch is really a 



persistent spiracular pseudobranch ; * hence it is probable that a 



like significance must be attached to this singular structure in 



other Teleosts. The evidence of the cranial nerves on this point 



is conflicting. If the pseudobranch pertains 



to the spiracular cleft its nerve supply 



should be derived from the nerve of that 



cleft viz. the seventh or facial nerve ; but 



if it represents a hyoidean hemibranch, 



then one would expect it to be innervated 



by the ninth or glossopharyngeal nerve. 



As a matter of fact, however, the organ is 



said to be supplied by the seventh in some 



Teleosts, and in others by the ninth 



nerve. 



In the Dipnoi the branchial system is 

 best developed in Neoceratodus, the increas- 

 ing importance of the lungs as respiratory 

 organs in Protopterus and Lepidosiren being 

 associated with a corresponding reduction 

 in the structural and functional develop- 

 ment of the gills. There is 110 trace of 

 spiracles in the adult. 



In Neoceratodus 2 there are five branchial 

 clefts, including the hyobranchial. Each 

 of the first four branchial arches carries a FIG. 

 pair of hemibranchs, and, as in the Holo- 

 cephali, the gill-lamellae are attached along 

 nearly their whole length to a well- 

 developed interbranchial septum (Fig. 165). 

 A peculiarity of Neoceratodus, which has no 

 counterpart in any other Fishes, is the 

 extension of the branchial lamellae on to 

 the dorsal and ventral walls of the branchial clefts, so that the 

 hemibranchs on opposite sides of each cleft are continuous both 

 dorsally and ventrally (Fig. 166). The fifth arch is gill-less. 



1 F. Maurer, Morph. Jahrb. ix. 1884, p. 229 ; xiv. 1888, p. 175. 



2 Giinther, Phil. Trans, clxi. 1871, p. 511 ; Baldwin Spencer, Macleay Memorial 

 Volume, 1892, p. 1. 



-Transverse sec- 

 tion through a branchial 

 arch of Xeoceratodus 

 (semi - diagrammatic). 

 a.b.tt. Afferent branchial 

 artery ; b.a, branchial 

 arch ; b.f, branchial 

 filaments ; e.b.v, efferent 

 branchial vessel ; g.r, 

 gill-rakers. (From Bald- 

 win Spencer. ) 



