116 THE SNAIL. 



the body- wall reduces this orifice to a small round aperture 

 on the right side, the respiratory or pulmonary aperture, 

 through which air alternately enters and leaves the cavity 

 during respiration. 



In close relation with the mantle-cavity are the rectum, 

 the heart, and the kidney and ureter. 



Place the snail in a dissecting dish wilder water, and pin 

 it firmly doion through the foot, tvith the dorsal surface 

 upward. Open the mantle-cavity by a transverse incision 

 through its thin roof, immediately behind the collar. Gut 

 backwa/rds from each end of the incision along the sides of the 

 cavity to its posterior end, cutting along the left side close to 

 the junction of the mantle and body-wall, and along the right 

 side about a quarter of an inch to the left of the prominent 

 ridge that borders this side, midway between this and the 

 right border of the kidney. 



Turn back the roof of the mantle-cavity, and pin it doion 

 so as to fully expose the pericardium and kidney. 



A. The Walls of the Mantle-cavity. 



1. The roof and sides of the cavity are formed by the 



mantle itself, a thin fold of skin which bears on its 

 under surface a very rich network of-^Iarge blood- 

 vessels, especially abundant in front and along the 

 right side. 



These vessels, in which the blood is aerated, unite 

 to form a large trunk, the pulmonary vein, which 

 takes back the oxygenated blood to the heart, run- 

 ning along the left side of the kidney. 



The mantle is greatly thickened in front to form 

 the collar : posteriorly it is thickened by the kidney 

 and pericardium, both of which lie in its substance. 



2. The floor of the cavity, which is formed by the dorsal 



wall of the body, is muscular, and strongly arched 

 from side to side. Contraction of its muscles causes 

 flattening of the floor, and protrusion of the body 

 further out of the shell, and thus enlarges the cavity, 



