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 lacunae 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. 



