154 FISHES. 



over the rudimentary liver. Of all other fishes, only in 

 Myxinoids the portal vein is contractile. All the blood- 

 corpuscles of Branchiostoma are colourless and without 

 nucleus. 



In Dipnoi a rudimentary division of the heart into a right 

 and left partition has been observed ; this is litnited to the 

 ventricle in Ceratodus, but in Zepidosiren and Frotopterus an 

 incomplete septum has been observed in the atrium also. 

 All Dipnoi have a pulmonal vein, which enters the atrium 

 by a separate opening, provided with a valve. The pulmonal 

 artery rises in Lepidosiren and Frotopterus from an arch of the 

 aorta, but in Ceratodus it is merely a subordinate branch, 

 rising from the Arteria coeliaca. 



