xx] 



CIRCULATOKY SYSTEM 



505 



to the left of the pharynx and opens into it in the mid-ventral 

 line as in Polypterini. In Ceratodus a similar duct leads from 

 the undivided sac to the pharynx. But the power of using the air- 

 bladder to breathe air is one which is shared by the Protospondyli, 

 the Aetheospondyli, and the Polypterini : the really characteristic 

 features of Dipnoi are to be found in the modifications of the 

 vascular system, which resemble closely those found in the 



^Poster/or 

 Carotid 



Anterior 



Carotid f^ EpibranchiaJ 

 essds 1-4 



FIG. 247. Diagram of the arterial arches of Ceratodus, 

 viewed from the ventral side, 



lowest land animals, the Amphibia, and strongly support the view 

 that Dipnoi in most points retain the structure of that group of fish 

 from which land animals were "derived. 



Thus we find that the atrium of the heart is divided by a 

 septum into a large right and a small left auricle, and that whereas 

 the right auricle receives its blood from the sinus veuosus, the left 

 receives blood direct from the lobes of the air-bladder, or as we 



