254 DIPNOI. 



posterior of these divides to form the afferent arteries to the 

 3rd and 4th branchial arches, on which a double row of gill- 

 filaments are always present ; it arises somewhat more dorsally 

 than the two anterior vessels which go to the 1st and 2nd 

 branchial arches, which are without gills in Proloplerus. The 

 anterior of these, viz. that to the first branchial arch sends a 

 branch to the hyoid gill in Protopterus. The efferent vessels of 

 each side unite to form one vessel (root of dorsal aorta), which 

 joins its fellow to form the dorsal aorta. The carotid arteries 

 pass off from the efferent vessel of the hyoid gill which joins the 

 aortic root, and the pulmonary arteries arise from the roots of 

 the aorta. 



In Ceratodus the hyoid gill is a pseudobranch and is supplied by a vessel 

 from the ventral end of the efferent vessel of the first branchial arch ; its 

 blood is returned into an artery called the anterior carotid, which goes to 

 the head. The pulmonary artery also arises from the efferent vessel of 

 the fourth branchial arch. 



The anterior part of the venous system is constructed on the 

 usual piscine type. In the posterior part there are some remark- 

 able peculiarities which recall Amphibia. On the left side there 

 is a posterior cardinal which arises in the left kidney and empties 

 into the left ductus cuvieri ; on the right side there is a vein 

 which may be the right posterior cardinal, but which is called 

 the inferior vena cava, which indeed it closely resembles ; it 

 arises in the right kidney, passes anteriorly through the liver, 

 receives in Protopterus the hepatic veins, and opens directly 

 into the sinus venosus. The kidney is supplied by the dorsal 

 aorta and by the caudal vein. . 



In Ceratodus the hepatic veins fall into the sinus venosus, and there is 

 an anterior abdominal vein formed by the union of a branch from each 

 iliac vein (from the pelvic limb). It empties directly into the sinus 

 venosus. Moreover in Ceratodus the left posterior cardinal and inferior 

 vena cava are direct continuations of the two branches into which the 

 caudal vein bifurcates ; they receive from, but do not give blood to, the 

 kidney. The venous blood to the kidney is partly derived from 

 veins of the posterior body wall and partly from a branch of the iliac vein. 

 In Ceratodus and possibly in all Dipnoi there is, as in Elasmobranchii, a 

 sub-intestinal vein which lies along the spiral valve and joins the portal 

 vein. 



The brain* presents the following features (Fig. 132). The 



* Burckhardt, Das Centralnervensystem von Protopterus annectens, Berlin 

 1892. Kerr, Q.J.M.8., vol. 46, 1903, p. 428. 



