CHAPTEK IV 



THE EVIDENCE OF THE RESPIRATORY APPARATUS 



Branchiae considei*ed as internal branchial appendages. — Innervation of branchial 

 segments. — Cranial region older than spinal. — Three-root system of cranial 

 nerves, dorsal, lateral, ventral. — Explanation of van Wijhe's segments. — 

 Lateral mixed root is appendage-nerve of invertebrate. — The branchial 

 chamber of Ammocoetes. — The branchial unit, not a pouch but an 

 appendage. — The origin of the branchial musculature. — The branchial 

 circulation. — The branchial heart of the vertebrate. — Not homologous with 

 the systemic heart of the arthropod. — Its formation from two longitudinal 

 venous sinuses. — Summary. 



The respiratory apparatus in all the terrestrial vertebrates is of the 

 same kind — one single pair of lungs. These lungs originate as a 

 diverticulum of the alimentary canal. On the other hand, the 

 aquatic vertebrates breathe by means of a series of branchiae, or gills, 

 which are arranged segmentally, being supported by the segmental 

 branchial cartilaginous bars, as already mentioned in the last chapter. 

 The transition from the gill-bearing to the lung-bearing vertebrates 

 is most interesting, for it has been proved that the lungs are formed 

 by the modification of the swim-bladder of fishes ; and in a group 

 of fishes, the Dipnoi, or lung-fishes, of which three representatives 

 still exist on the earth, the mode of transition from the -fish to the 

 amphibian is plainly visible, for they possess both lungs and 

 gills, and yet are not amphibians, but true fishes. But for the 

 fortunate existence of Ceratodus in Australia, Lepidosiren in South 

 America, and Protopterus in Africa, it would have been impossible 

 from the fossil remains to have asserted that any fish had ever 

 existed which possessed at the same moment of time the two kinds 

 of respiratory organs, although from our knowledge of the develop- 

 ment of the amphibian we might have felt sure that such a transitional 

 stage must have existed. Unfortunately, there is at present no 

 likelihood of any corresponding transitional stage being discovered 



