Vascular System of Bdellostoma dombeyt. 15 
behind this duct usually projects as a lateral pocket into the 
right pericardial cavity. The auricle is attached ventrally 
and internally (z. e., to the left) by the mesocardium. ‘The 
anterior and posterior extremities of the auricle, however, 
project freely into the over-lying pericardial cayity. 
The cavity of the auricle (Fig. III) is very irregular. 
Beside the three pocket-like spaces (anterior, posterior, and 
lateral), the cavity is made irregular by a network of mus- 
cular trabeculze which project from the walls, and sometimes 
across the cavity. The auricle has two openings. The 
afferent, or sinu-auricular aperture, is in the posterior por- 
tion of the floor. On viewing the floor from above, the long 
narrow slit-like opening is seen extending diagonally from 
side to side. The opening is guarded by two thin mem- 
branous valves (Fig. III, sav), whose free margins readily 
allow the influx of blood, but effectually prohibit its reflux. 
A short distance anterior from the sinu-auricular opening we 
find the auriculo-ventricular aperture, an oval foramen, on 
the right side in the duct previously described. This aper- 
ture is also guarded by a pair of valves, semi-lunar in shape; 
one lying anterior and the other posterior to the opening. 
These valves are smaller but stronger than the sinu-auricular 
valves. During the flow of blood from the auricle into the 
ventricle they lie flattened against the sides of the duct, with 
their free margins extending toward the ventricle and offer- 
ing no resistance to the flow of blood. During the ven- 
tricular systole the valves prevent the reflux of blood by the 
apposition of their free margins in the median line, com- 
pletely closing the channel. 
The ventricle (Figs. I, II, III, V) is an ovoidal sac, slightly 
elongated and flattened dorso-ventrally. The lateral margin 
on the left side, next to the auricle, is less convex than that 
on the right. The ventricle is nearly in the median line 
of the body, just below the alimentary canal. It lies to 
the right of the auricle, somewhat ventral and anterior to it. 
The wall of the ventricle is much thicker than that of the 
auricle, and encloses a cavity shaped like a Scottish bag-pipe 
(See Fig. III). The inner surface of the wall, like that of the 
auricle, is marked by projecting muscular trabeculae. There 
are two openings into the ventricular cavity. The afferent, 
3 
