39 



valves preventing the reflux of the blood into the auri- 

 cles during the ventricular systole. On the floor of the 

 pericardium, beneath the ventricle, are the openings of the 

 reno-pericardial canals. On the anterior part of the floor 

 are several openings which are the terminal portions of 

 systems of tubules forming the paired pericardial glands. 

 The tubules are lined with cells containing brownish-red 

 concretions, and are distributed over a wedge-shaped area of 

 the mantle extending, ventrally, from the most dorsal 

 portion.* The ventricle is prolonged backwards into a 

 short narrow neck which still contains the rectum. A 

 transverse membranous partition, beginning at the dorsal 

 surface of this neck, extends backwards and downwards 

 across its cavity, embracing the rectum, and ends so that 

 its free edge lies near the ventral surface. This must form 

 a valve preventing the reflux of blood into the ventricle 

 from the posterior part of the body. Behind this valve the 

 tube expands forming a "bulbns arteriosus" (B.a., fig. 30) 

 with thin muscular walls. This terminates in two lateral 

 branches, the right and left posterior pallial arteries which 

 diverge from each other and pass backwards beneath the 

 posterior adductor. Here their walls become very ill- 

 defined and communicate freely with a system of lacunas 

 between the bundles of the muscle. The arteries, which 

 are now difficult to trace, reach the mantle margin and 

 terminate in the sinuses there and in the walls of the 

 siphons. 



Anteriorly the ventricle passes into a single median vessel 

 with well defined walls, the anterior aorta (Ao., fig. 30); 

 this pierces the posterior wall of the viscero-pedal mass, and 

 travels along in the dorsal region of the latter, giving off, 

 in its course, small vessels to the digestive gland. Near 



* Grobben, Dr. C. Die Pericaidialdiiise der Lamellibranchiaten. Albeit. 

 Zool. Inst. Wien. Bd. VII. 1888. 



