Vascular System of Bdellostoma dombeyt. 19 
lobe of the auricle projects. The roof of the inner chamber 
is in contact with the ventral surface of the alimentary canal. 
The floor lies upon the dorsal surface of the anterior lobe of 
the liver. The anterior wall lies against the connective 
tissue surrounding the last gill pouch on the right side. The 
external wall is formed by the portal septum, which is incom- 
plete ventrally, leaving an extensive communication with the 
outer chamber. The inner wall is formed by the mesocar- 
dium, which is also incomplete. Above the mesocardium is 
a narrow longitudinal slit (about 1 cm. in length) through 
which the right and left pericardial cavities communicate. 
The ventricle of the heart occupies the anterior portion of 
the inner chamber. ‘The double-layered mesocardium passes 
upward and outward from its attachment, soon dividing into 
two sheets which form the inner pericardial layer immediately 
surrounding the ventricle. (Fig. VI.) Anteriorly the inner 
pericardial layer becomes continuous with the outer, forming 
no pericardium. In this way the anterior end of the ventricle 
is not covered by the pericardium, but lies oufszde the pericar- 
dial space. The mesocardium is attached ventrally to the 
left margin of the anterior lobe of the liver. Posteriorly it 
encloses the sinus and veins opening into it. 
The left pericardial cavity lies on the left side of the meso- 
cardium, somewhat dorsal to the right cavity. It is a small 
elongated sac, which closely surrounds the auricle. The roof 
is in contact with the alimentary canal internally and the 
dorsal body wall externally. The floor and external walls 
touch the latero-ventral body wall. The inner wall is formed 
by the incomplete mesocardium, leaving the right and left 
pericardial cavities in communication dorsally. The exterior 
wall abuts against the postero-dorsal wall of the oesophago- 
cutaneous duct. Posteriorly, the cavity ends as a short blind 
pocket. From the roof the left pronephros and the left 
anterior cardinal vein project into the cavity, pushing the 
pericardium before them. The mesocardium extends into the 
left pericardial cavity and surrounds the ventricle on the 
right side. The postero-internal angle of the auricle often 
extends through the slit-like foramen above the mesocar- 
dium, and lies in a pocket within the right pericardial cavity. 
This relation is not constant, however. 
7 
