124 THE SNAIL 



giving off large branches to the salivary glands and 

 body-wall. It then continues its course forwards 

 along the under surface of the oesophagus ; pierces 

 the sub-oesophageal ganglionic mass, passing between 

 the pedal and viscero -pleural ganglia; gives off a 

 large pedal artery which runs back along the foot ; 

 and finally divides into large branches supplying the 

 tentacles and buccal mass. 



2. The venous system. 



The venous system consists partly of tubular 

 vessels with definite walls, and partly of irregular 

 lacunar spaces, the exact relations of which to the 

 arteries and to the body-cavity are not determined. 



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 

 spire 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 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 reach- 

 ing the auricle, the pulmonary vein receives a large 

 renal vein from the kidney. 



