CERATODUS 85 



polluted, and the animal rises to the surface to take in air 

 through the nostrils. It is this capacity of breathing by means 

 of lungs and gills which is responsible for the name of the 

 group Dipnoi, to which Ceratodus belongs. The lung is 

 homologous with the swim-bladder of Gadus. 



Vascular System. — Blood is supplied to the lung by 

 branches of the last (6th) efferent branchial artery, which can 

 now be called pulmonary arteries. The right pulmonary 

 artery runs direct to the lung, but the left passes down under 

 the gut and up again on the right side parallel with the windpipe 

 or trachea. This shows that the primitive position of the lung 

 was ventral, and that it moved up the right side, whither it is 

 followed by the left pulmonary artery. Blood leaves the lung 

 by the pulmonary veins, which unite to form one vein. This 

 vein also passes down on the right side of the gut and goes 

 right through the sinus venosus to open into the left side of 

 the auricle. The auricle itself is partially divided into two by 

 a septum, so that the blood (oxygenated) from the lung comes 

 in on the left, and that from the rest of the body (deoxygenated) 

 enters on the right from the sinus venosus. The ventricle is 

 single, but the conus arteriosus is nearly divided into two by 

 enlarged valves. 



The ventral aorta leads forwards from the conus and gives 

 off four pairs of afferent branchial arteries. The two collecting 

 vessels in each gill-arch join to form the efferent branchial 

 arteries which combine to form the dorsal aorta. The arrange- 

 ment of the valves in the conus and truncus arteriosus is such 

 that the blood from the sinus venosus tends to go into the 

 posterior branchial arches (and so to the pulmonary arteries), 

 while that from the pulmonary vein gets into the anterior 

 arches. There is therefore an attempt to separate the circula- 

 tion of the freshly-oxygenated blood from that of the impure 

 blood which should go and be oxygenated. 



There are two important points to notice in the venous 

 system. The posterior cardinal on the right side loses its 

 connexion with the ductus Cuvieri. Instead, it has developed 

 a new connexion with the sinus venosus, forming the inferior 

 vena cava. The ductus Cuvieri can also be called the superior 



