Spencer— Zhe Blood Vessels of Ceratodus. 21 
The large hepatic portal vein leaves the intestine and enters the liver by the 
side of the enteric artery (fig. 9, 2. . v). 
It is formed by the union of three vessels :— 
(1) The dorsal-intestinal vein.—For the greater part of its course this is a 
double vessel one lying close to either side of the dorsal-intestinal artery. At the 
posterior end of the intestine these vessels diverge and pass to the sides of the latter, 
Where the spiral fold first passes dorsally across the intestine they unite together 
and the single vessel thus formed runs backwards by the side of the corresponding 
artery following the line of attachment of the fold to the ventral surface (figs. 8, 
9 and 12, d. znzt. v.). 
(2) The ventral-intestinal vein.—This is a single vessel lying to the right side of 
the corresponding artery and running along the length of the intestine (v. z/. v.). 
(8) The cxtra-cntestinal vern.—This passes along the thickened free edge of the 
spiral fold, gradually disappearing before the posterior end of the latter is reached 
(tnt. int. v.). 
The three vessels above described, the relative positions of which are indicated 
diagrammatically in fig. 12, unite to form the large hepatic portal vein which enters 
and breaks up into branches in the liver. 
The dorsal and ventral intestinal veins receive branches which correspond in 
position to the attachment of the spiral fold to the wall of the intestine as in the 
case of the arterial branches previously described. The arrangement of these is 
represented in fig. 9 and on the right side a special longitudinal vein is developed 
in connection with a curious glandular structure present on the wall of the intestine. 
Each also receives special branches from the walls of the anterior portion of the 
intestine. 
The hepatic veins are two large vessels receiving the greater part of the blood 
from the liver. They enter the sinus venosus (fig. 17), one on erther side of the median 
inferior vena cava, the right hepatic vein lying nearer to this than the left one. 
E. Derivatives of the anterior cardinal system. 
On either side of the body a large Ductus Cuvieri is formed by the union of the 
veins entering the heart, other than the hepatic and the inferior vena cava. The 
two pass inwards through the dorsal and posterior angles of the pericardium (fig. 1%); 
and open into a large median sinus venosus, which may indeed be said to be formed 
by their union, together with the terminal portion of the inferior vena cava. 
