16 Cincinnati Society of Natural History. 
or auriculo-ventricular is an oval foramen situated near the 
posterior end on the inner side. It corresponds to the end of 
the short ductus auricularis already described. The foramen 
ordinarily measures about a millimeter in diameter in indi- 
viduals of fifty centimeters length. The afferent or aortic 
aperture lies at the anterior extremity of the ventricle. It is 
nearly circular in outline and is likewise guarded by a pair of 
strong semi-lunar valves, one dorsal and one ventral. These 
are the strongest and thickest of the cardiac valves. They 
contain much elastic tissue, and are continuous with the 
walls of the aorta at its base. Their action is similar to that 
of other valves. 
PERICARDIAL CAVITY. 
The alimentary canal in Bdellostoma is suspended from the 
mid-dorsal line of the body wall by a mesentery, which is 
formed by the reflection of the lateral peritoneal sheets. (See 
Fig. IV.,m.). Ventral to the alimentary canal, in the anterior 
abdominal region, the liver is likewise enclosed by a contin- 
uation of the same mesentery. (See Fig. V, L). Above the 
anterior lobe of the liver, the lateral peritoneal sheets do not 
meet above the alimentary canal to form a distinct mesentery. 
They are widely separated, and in the space between them 
‘the pericardial cavity is formed, just above the anterior lobe 
of the liver. The relations of the peritoneal and pericardial 
cavities are shown in diagram VI, which represents a cross- 
section through that region. For convenience of descrip- 
tion, we may conceive of the pericardial sac as formed by an 
evagination of the peritoneal membrane on the right side 
into the space above the anterior lobe of the liver, and below 
the alimentary canal. The sac formed by this evagination is 
in turn zxvaginated by the pushing in of the heart (V) from 
below. In this way the pericardial membrane becomes 
arranged to form a double wall made up of two layers — an 
outer (O), or parietal layer, which encloses the pericardial 
cavity (pec), and which is reflected to form the inner, or 
visceral layer (1), which immediately invests the heart. By 
the approximation of the pericardial layers, where they are 
reflected over the heart, a ventral cardiac ligament, the meso- 
cardium (cm) is formed. The mesocardium partially sep- 
arates the general pericardial cavity into two unequal divi- 
4 
