CIRCULATORY SYSTEM. 139 



Two large irregular pedal sinuses lie in the foot, 

 one on each side of the pedal gland. 



A visceral sinus commences at the top of the 

 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 pul- 

 monary vessels : these run parallel to each other 

 in the roof of the mantle-cavity, interdigitating with 

 the efferent vessels, with which- they 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 reaching the auricle, the pulmonary vein 

 receives a large renal vein from the kidney. 



