CHAPTER 1V 
THE EVIDENCE OF THE RESPIRATORY APPARATUS 
Branchie considered as internal branchial appendages.— Innervation of branchial 
sepments.—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 Ammoccetes—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 branchiz, 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 
