LUNG-FISHES 



647 



till in the adult Dipnoan it comes to lie dorsal to the gullet, 

 but still communicates with the mouth cavity by a ventrally 

 placed glottis. 



A pulmonary artery rises on each side from the last 

 (fourth) epibranchial artery to 

 supply blood to the lung. Cor- 

 related with this the chambers 

 of the heart are divided, though 

 imperfectly, so that pulmonary 

 and systemic circuits are in 

 some measure separate. This is 

 helped by a directive system of 

 valves in the conus. The two 

 circuits are, however, not shut 

 off from one another as in higher 

 vertebrates, and a certain amount 

 of admixture takes place. In the 

 venous system the greater part 

 of the blood from the hind body 

 and kidneys passes to the sinus 

 venosus by a shortened route via 

 the liver, by a vein resembling 

 the inferior vena cava of higher 

 forms which supersedes the pos- 

 terior cardinal sinus of the right 

 side. 



In the brain the cerebral 

 hemispheres are relatively large, 

 compared with the very small 

 optic lobes and cerebellum. In 

 Lepidosiren at least the hemi- 

 spheres show an interesting 

 feature in the arrangement of 

 the ganglion cells in layers to 

 form a definite cortex in certain 

 regions. 



In early life the nostrils are in the form of two grooves 

 placed just above the mouth and extending outwards 

 from it. During development each groove is bridged, 

 leaving only an opening at each end — anterior and posterior 

 nasal openings. In the development of higher vertebrates, 



Fig. 372. — Skeleton of Cera- 

 -. todus fin. — From Gegen- 

 baur. 



a., Central axis; r., raclials; 

 /;., basal piece. 



