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 

 (Gunther [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 



