248 



DIPNOI 



no branchial rays on the 

 branchial arches. Gill -rakers 

 with cartilaginous supports are 

 present. 



An opercular fold from the 

 hyoid arch completely covers 

 the gill-clefts ; and in the Dip- 

 neumones the external opening 

 is narrowed down to a small 

 aperture. 



As the importance of the 

 gills diminishes that of the air- 

 bladder increases. As described 

 above (p. 223), it is morpho- 

 logically ventral, arising as a 

 median ventral outgrowth of 

 the oesophagus (Fig. 217). 

 While the muscular glottis re- 

 mains ventral, the posterior 

 saccular dilatation shifts round 

 to a dorsal position. It is highly 

 vascular and functions as a lung 

 (Giinther [190], Spencer [413]), 

 the deeply folded inner wall 

 being cellular in structure. 



The vascular system is of 



great interest, showing many 



points of resemblance to that 



of the Amphibia as well as 



primitive characters (Owen 



311], Lankester [277], Boas 



38], Spencer [413], Parker 



324], Kellicott [257], Rose 



373]). The heart, unlike that 



of other fish, is in a thin-walled 



pericardium. It is completely 



twisted, so that the sinus venosus 



lies dorsal to the atrium which 



passes in front of the conical 



ventricle (Figs. 218-19). The 



sinus receives all the venous 



systemic blood from the two 



ductus Cuvieri, the vena cava 



posterior, and the hepatic veins. 



But the pulmonary vein, coming 



