102 THE ANATOMY OF VERTEBRATED ANIMALS. 



Except in AmpMoxus, the branchice are always lamellai', 

 or filamentous, appendages of more or fewer of the visceral 

 ai'ches ; being sometimes developed only on the proper bran- 

 chial arches, sometimes extending to the hyoidean arch, or 

 (as would appear to be the case with the spiracular bi-anchise 

 of some fishes) even to the mandibular arch. The branchiae 

 are always siippHed with blood by the divisions of the 

 cardiac aoi-ta; and the diiFerent tnmks which cai-ry the 

 aerated blood away, unite to form the subvertebral aorta, 

 so that all vertebi-ated animals with exclusively branchial 

 respiration have the heai-t fi^lled with venous blood. 



In the early life of many branchiated Vertebrata, the 

 branchise project freely from the visceral arches to which 

 they are attached, on the exterior of the body ; and in. some 

 Amphibia, such as the Axolotl {Siredon), they retain their 

 form of external plumelike appendages of the neck through- 

 out life. But in the adult life of most Fishes, and in the 

 more advanced condition of the Tadpoles of the higher 

 Amphibia, the branchise are internal, being composed of 

 shorter processes, or ridges, which do not project beyond 

 the outer edges of the branchial clefts ; and, generally, 

 laecome covered by an operculum developed from the second 

 visceral arch. 



The lungs of vei-tebrated animals are sacs, capable of 

 being filled with air, and developed from the ventral wall of 

 the pharynx, with which they remain connected by a shorter 

 or longer tube, the trachea, the division of this for each 

 lung being a broticlms. Venous blood is conveyed to them 

 directly from the heari by the pulmonary arteries, and 

 some * or all of the blood which they i-eceive goes back, no 

 less directly, to the same organ by the pulmonaiy veins. 



The vascitlar distribution thus described constitutes an 

 essentia] part of the definition of a hing, as many fishes 

 possess hollow sacs filled with air ; and these sacs are de- 

 veloped, occasionally, from the ventral, though more com- 

 monly from the dorsal, wall of the pharynx, oesophagus, or 



* Generally all, but in some of the blood supplied to the lungs 

 Amphibia, such as Proteus, part enters the general circulation. 



