IV.J THE COMMON SNAIL. 295 



cutting close alongside the rectum, in order that that 

 portion of it bearing the smaller pulmonary vessels 

 may be reflected intact. 



Note in' order : 



a. The circulus venosus pulmonis ; a great sinus sur- 

 rounding the base of the pulmonary sac. 



b. The marginal sinus of the visceral sac; a spacious 

 channel continuous with a. posteriorly and running 

 within the thickened edge of the visceral sac. 



On exposing the interior of the pedal sinus in an 

 uninjected specimen, there will be seen opening 

 into it the orifices of adjacent lacunae. During the 

 process of injection from the above sinus the fluid 

 introduced passes through these — partly into the 

 body-wall, partly into the "body-cavity^-in either 

 case finally reaching the pulmonary circulus, to- 

 wards which all those channels in which venous 

 blood circulates eventually converge. 



c. The afferent pulmonary vessels; arising from the pul- 

 monary circulus on all sides. They alternate with 

 striking regularity with the efferent pulmonary 

 vessels, the two sets of trunks being connected by 

 a well-differentiated respiratory plexus. 



The afferent pulmonary vessels of the left side 

 and front wall of the lung-sac are elongated. 

 Those of its right side are very short, but in so far 

 as their relations to the lung-sac are concerned 

 they differ from their fellows in no other respect ; 

 after leaving that to reach the heart however, they 

 enter the excretory organ and break up within it 

 into a renal plexus, from which the efferent renal 

 vessels previously described (Sec. H. 1 b) arise. 



