116 INVERTEBRATE ZOOLOGY 
then make an incision in the mantle wall from this opening 
following the collar round the outer side of the whorl to the 
heart; continue the incision across the artery leading out of 
the heart, and through the delicate membrane between the liver 
and the kidney to the rectum, at the inner border of the whorl. 
The mantle can now be laid back and its cavity with the organs 
exposed. The broad rectum will be seen running along the 
entire inner border of the mantle cavity. Make, now, an addi- 
tional incision from the respiratory pore along the inner (lower) 
border of the rectum as far as the kidney. Lay back the mantle 
and pin it down as flat as possible under water. Identify the 
heart within the pericardium, the kidney, and the rectum. 
The respiratory and circulatory systems. Observe the lung, the 
network of blood vessels in the inner surface of the mantle, and 
the large pulmonary vein, which runs along the kidney to the 
heart. Slit open the pericardium. The two chambers of the 
heart will be more distinctly seen, the thin-walled auricle into 
which the vein runs and the larger ventricle. Back of the latter 
the aorta passes into the viscera; its cut end will be seen. 
The process of respiration and circulation is the following: 
the air is drawn into the mantle cavity through the respiratory 
pore; this is accomplished by the alternate enlarging and con- 
tracting of the cavity by means of the muscular body-wall which 
constitutes its floor. Notice the longitudinal and the trans- 
verse muscles in this floor. The blood circulating in the lung 
is oxygenated and passes into the heart through the pulmonary 
vein as arterial blood. It is forced by the heart through the 
aorta, and thence through arteries to all parts of the body, 
whence it returns through blood lacune to the lung. 
The excretory system. The large kidney has already been seen. 
It is a sac, the glandular projections of the walls of which 
almost fill its lumen. As is the case with pelecypods, the 
kidney communicates with the pericardial space through a fine 
canal and also with the mantle cavity by means of a ureter. 
