Vascular System of Bdellostoma dombeyt. 17 
sions, right and left. These two cavities communicate freely 
with each other dorsally (above and behind the heart), but 
are entirely separated ventrally. The right pericardial cavity, 
which is the larger, remains in free communication with the 
peritoneal cavity through the pericardio-peritoneal foramen 
(pef). The heart has therefore exactly the same relation to 
the pericardium and the pericardial cavity as the intestine has 
to the peritoneum and the peritoneal cavity. There is only 
this difference, that the mesentery is dorsa/ to the intestine, 
while the mesocardium is ventral to the heart. 
The right pericardial cavity may be described approxi- 
mately as an elongated lenticular cavity, 2-3 cm. long, flat- 
tened dorso-ventrally, and lying just above the anterior por- 
tion of the anterior lobe of the liver. The anterior end of 
the cavity is larger and extends somewhat toward the right. 
It includes that portion of the cavity which contains the ven- 
tricle. The posterior end of the cavity is narrower, and 
extends backward and toward the left. The right pericardial 
cavity, as a whole, is bounded above by the dorsal body wall; 
below by the anterior lobe of the liver; internally by the 
mesocardium and the intestine (which it partially surrounds); 
and externally by the reflected peritoneum, which lies for the 
most part against the dorso-lateral body wall. An imperfect 
septum, the jortal septum, complete only in the posterior 
region, is formed by the reflection of the roof into the right 
pericardial cavity. It extends parallel to the long axis of the 
cavity, and partially divides it into two chambers — a smaller 
outer and a larger zmner chamber. ‘This double-layered portal 
septum in the middle portion surrounds the portal heart; 
anteriorly, the anterior portal vein runs between its walls; 
and posteriorly it surrounds the common portal vein. 
The outer chamber of the right pericardial cavity lies 
external to the portal septum. It isa narrow cavity, about 
2-3 cm. in length, lying parallel to the inner chamber. It com- 
’ municates with the inner chamber below the septum through- 
out almost its entire length. Only the extreme posterior 
portion of the outer chamber, into which the lateral wall of 
the alimentary canal projects, is completely separated by the 
septum from the inner chamber. In the posterior region the 
outer chamber communicates laterally and externally with 
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