108 THE SNAIL 



mass, grows forwards along the back, ending in front in a 

 thickened rim, the collar. Between the mantle-fold and the 

 back of the animal, is the large mantle-cavity, which at first 

 opens anteriorly by a wide orifice. This orifice becomes 

 reduced, by fusion of the collar with the body- wall, to a 

 small round aperture placed 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 under water, and pin 

 it firmly down through the foot, with the dorsal surface 

 upwards. Open the mantle-cavity by a transverse incision 

 through its thin roof, immediately behind the collar. Extend 

 this incision backwards along each side 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 doivn 

 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 large 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 arterialised blood to the heart, running 

 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 



