172 GUY C. ROBSON 



From the general venous sinus the blood passes (a) to the 

 renal portal system by more or less clearly- defined vessels, 

 (fe) to the gills through the rectal sinus. From the kidney the 

 blood passes into the portal vein (q.v.). The complete course 

 of the latter is not easy to trace. In one or two cases it was 

 found entering the auricle close to the root of the pulmonary 

 vein, though in other cases its junction is not so clearly seen 

 and it might even open into the pulmonary vein itself. It 

 is possible, however, that some of the blood in the kidney may 

 find its way into the gill directly, as the rectal sinus was adjacent 

 to the kidney on part of its course. The rectal sinus proper 

 appears to be cut off from the other venous sinuses, but to be 

 in communication with them by means of a loose lacunar 

 system. 



From the rectal sinus afferent vessels run to the gill and 

 pass along the base of the gill-lamellae (q.v.). The arterialized 

 blood is carried from the gill by the pulmonary vein. It leaves 

 the ventricle by the aorta, which divides into anterior and 

 posterior branches. The first-named runs forward along the 

 wall of the pericardium sending out branches in its course. 

 The posterior branch passes between the stomach and intestine 

 and probably enters a lacuna in this position. From the 

 lacuna is given off among other vessels a clearly-marked 

 genital artery which can be traced backwards to finer branches 

 distributed to the various processes of the gonad. 



(3) Eespiratory System. 



The gill (Text-fig. 6) is monopectinate and composed of 

 broadly-triangular plates hanging freely in the pallial cavity. 

 On the rectal side the extremity of each plate is free for a short 

 distance. 



The histology and structure correspond in general with that 

 of Bythinella, but Bregenzer does not indicate the relation 

 of the gill to the adjacent parts of the circulatory system. 



On the whole it would seem that in P. ventrosa the 

 apical portion of the lamellae is more elongate and triangular 

 in transverse section than in other forms. It should be 



