50 



The second focus of the venous blood is the renal sinus, 

 towards which all the blood circulating in the viscero- 

 pedal mass converges. There are two large sinuses in 

 the latter, an irregular sinus lying on the anterior margin 

 and a posterior pedal sinus (Sin. p.) lying beneath the 

 internal part of the posterior retractor muscles of the foot. 

 As the latter are gathered up into the two compact muscle 

 bundles which run upwards through the renal organ, they 

 leave an opening in the muscular wall of the foot which 

 leads into the two short longitudinal trunks, these, finally, 

 open into the large central sinus in the middle of the renal 

 organ (Sin.ren., figs. 7 and 30). 



From this central space the venous blood flows outwards, 

 bathing the renal secretory tubules in its course, and 

 enters two longitudinal vessels which run along the bases 

 of the ctenidia. These are the common afferent branchial 

 vessels (Br.aff., fig. 30). From them a series of vessels, 

 running perpendicularly to the bases of the ctenidia, enters 

 each branchia — the afferent branchial vessels {Br.aff'., figs. 

 26 and 30). These communicate through the interfila- 

 mentar branchial junctions with a series of similarly 

 disposed vessels — the efferent branchial vessels (Br.eff'., 

 figs. 26 and 30), which fall into a pair of common efferent 

 branchial vessels {Br.eff.), and these finally open into the 

 auricles. From the auricles the blood enters the ventricle 

 through the openings at the apices of the former, reflux 

 being prevented by the action of the valves guarding these 

 openings. 



Menegaux* bases a theory accounting for the protrusion 

 of the foot, siphons, and mantle edges, on the anatomical 

 relationships indicated above, i.e., the presence of a valve 



* Recherches Sur le Circulation des Lamellibranches Marins. 296 pp., 

 Besancon, 1890. Also Comptes Kendus de l'Acad. Sci., Paris. T. CVIII., 

 pp. 361—364, 1889. 



