388 BIOLOGY. 



and is therefore found only in Reptiles, Birds, and 

 Mammalia. 



Lungs. 



2225. In Fishes the internal organ of respiration is 

 already indicated by a csecal eversion of the oesophagus, 

 which is surrounded by the branchial arches. This mem- 

 branous csecal eversion is called the swimming-bladder, 

 which in the higher animals becomes double on account 

 of their symmetry, and is then called lung. 



2226. In the Fish the aquatic and aerial process of 

 respiration are present together, the former being exter- 

 nal, the latter internal. 



2227. The branchial arches having coalesced, are con- 

 verted in the higher animals into tracheal rings, into the 

 larynx and the posterior cornua of the hyoid bone, if 

 these are present, as in the Reptiles. The larynx is there- 

 fore no special organ, but only a remnant of the branchial 

 respiratory apparatus. 



2228. The laryngeal vessels are, like the thyroid 

 glands, branchial vessels, and in Fishes, therefore, the 

 branchial vessels do not correspond to the pulmonary 

 vessels, but to those of the trachea. The pulmonic ves- 

 sels of Fishes are the blood-vessels of the swimming- 

 bladder, which convey blood directly into the heart, 

 whereby this organ obtains the signification of the left 

 or arterial heart. 



2229. When the branchial apertures have coalesced, 

 the nose then opens- into the mouth or into the trachea, 

 and in this way the nostrils assume the complete cha- 

 racter of aerial foramina. 



2230. The nose is therefore originally an organ of 

 smell, and then a part of the respiratory system. It is 

 the animal lung. 



2231. As the secretion of bone is a product of the 

 more powerful process of oxydation, so do the bony 

 rings multiply beneath the branchial arches or the larynx, 

 and are called tracheal rings. In the feebly respiring 



