MANTLE-CAVITY. 125 



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, 

 causing an inrush of air through the pulmonary 

 aperture. On retraction of the body, the floor resumes 

 its previous convex shape, diminishing the size of 

 the cavity and effecting expiration. 

 k By alternate protrusion and retraction of the 

 body the air in the mantle-cavity is continually 

 renewed, and the cavity thus becomes a lung. 



B. The Organs in relation with the Mantle-cavity. 



1. The pericardial cavity and heart. 



a. The pericardial cavity, which has already been seen 



from the exterior, is oval, thin-walled, and half 

 an inch or more in length. It lies in the roof of 

 the mantle-cavity at its posterior end and left side, 

 and is in close contact with the left side of the 

 kidney about the middle of its length. 



Open the pericardial cavity to expose the heart. 



b. The heart has two chambers, of about equal size. 



i. The auricle is proximal, i.e. nearer the collar, 

 and has thin walls. It receives in front the 

 pulmonary vein, which returns to it oxygenated 

 blood from the lung. 



ii. The ventricle is the pear-shaped distal division 

 of the heart, with thick muscular walls. It 

 receives oxygenated blood from the auricle, 

 and drives it through the aorta to all parts of 

 the body. 



The blood-vessels will be described later on. 



2. The kidney and ureter. 



a. The kidney is a large pale yellow triangular body 

 in the posterior part of the roof of the mantle- 



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