BREATHING ORGANS. 



27^ 



the Urodela ; but in the Anura in whicli the gill-openings become 

 covered by a membranous operculum (fold of skin growing back 

 from the hyoid arch), the external gills atrophy and are replaced 



BR.2 



MaLhall)rFaffLent Vessel 0%''^^^^ °' V'l'^^'f ^^ '''' t''"^ of hatching x 40 (after 

 second Bif3 thM branchtl irnh J^ branchial arch ; BF fore brain; £/il first, BR2 

 dual amh HAAyom^n^t.T^^^^ ^^"e'^t vessel of first bran- 



archinephric ductofr?^hf,irtP^r^^^^^^ 'V^'?."^ ^^"^ ' ^^ infundibulum ; KA 



riglitpronenhros A's"ri^^^^^^^ °* left side ; A'P pronephros ; jPS tliird opening of 



L' P'?ary^^Tr^'rntesfiLl'r'eg^r/of ^e'nfXT r°?o^ "'' ' ^* °^''' ^'^^"^ ^ ^^ 



by a series of very similar but smaller structures developed 

 along the more ventral portions of all four branchial arches. 

 These are the internal gills, so called from their position beneath 

 the operculum and not from any essential difference between 



