CIRCULATOEY SYSTEM. 131 



spiral and runs down it, receiving branches from the 

 various organs, to the posterior end of the mantle- 

 cavity, where it opens into the pulmonary sinus, a 

 large vein running all round the mantle-cavity at the 

 junction of its sides and floor. Along the right side 

 the pulmonary sinus lies immediately ventral to the 

 rectum, to which it is closely attached, and from 

 which it receives numerous small veins. From the 

 pulmonary sinus the blood is carried to the roof of 

 the mantle cavity by the afferent pulmonary vessels : 

 these run parallel to one another in the roof of the 

 mantle-cavity, interdigitating with the efferent ves- 

 sels, with which thay are connected by a network of 

 thin-walled vessels in which the blood is aerated. 

 The efferent vessels unite to form the pulmonary 

 vein, a large trunk running straight back in the roof 

 of the mantle-cavity to the heart. Just before reach- 

 ing the auricle, the pulmonary vein receives a large 

 renal vein from the kidney. 



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