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by a pseudo-epithelium due to modified cells. The centre 

 of the venous sinus system is the region on the surface of 

 the digestive gland at the back of the pericardium and 

 underneath the renal organ (fig. 37). There is a large 

 sinus — the largest in the body — underneath the pallial 

 cavity (fig. 37), while an important vein runs along the 

 right margin of the mucous gland (figs. 35 and 38, 

 R. Sin.). These important regions may be named as 

 follows : — The sinus in the anterior perivisceral cavity 

 will be termed, the Cephalic Sinus (fig. 37, Ceph. Sin.), 

 as it collects blood from the head and foot. The branched 

 system, situated at the back of the pericardium, and 

 under the renal organ is the Renal Sinus System (fig. 37), 

 while the long vessel-like sinus which extends from the 

 right side of the renal organ to the anterior end of the 

 mucous gland is the Reno- Mucous vessel (figs. 38 and 35, 

 R. sin.). 



Blood returning from the viscera (digestive gland 

 and gonad) passes by means of superficial vessels (fig. 35) 

 to a sinus which extends along the columellar surface of 

 the visceral mass. This sinus stretches as far as the 

 region of the pericardium and there becomes connected 

 with the renal sinus system (fig. 37), by means of which 

 the blood reaches the renal organ. 



Blood returning from the head, proboscis, and foot 

 flows into the great cephalic sinus (fig. 37, Ceph. Sin.), 

 and from here two paths may be taken. These leave the 

 perivisceral cavity at the posterior extremity of the 

 mantle cavity, and are close together. One ascends and 

 reaches the renal organ directly. It passes along its left 

 side, internally, so that it cannot be seen unless the renal 

 organ is turned back as in figure 37. The other receives 

 blood from the oviduct and rectum (figs. 37, 38) and enters 

 the Renal Sinus System and so to the renal organ. 



