82 OKHAMANDAL MARINE ZOOLOGY REPORT 



anterior face the first section of the right branchial mesentery is hung, and in like 

 manner the fore part of the left branchial mesentery is suspended from the face of the 

 left renal tube. 



The extremely short internephridial passage lies transverse to the median plane, 

 bounded dorsally by the visceral mass, ventrally by the adductor muscle, and anteriorly 

 by that ventrally-directed lobe of the visceral mass which pushes its way downwards 

 and forwards between the auricles and apparently through the nephridia in its 

 accompaniment of the pyloric caecal sac. Hence in serial sections we see the inter- 

 nephridial passage (fig. 24, I.n.p.) as a wide transverse channel connecting the two 

 nephridia immediately behind an " island " of reproductive tissue having a cross 

 section of the pyloric caecum in the centre, and one of the ventral trunk of the 

 aorta to the left side. 



Posteriorly each nephridium is prolonged in a posterior ventral direction as a csecal 

 prolongation ; that on the left very short, that on the right relatively of great length. 

 The latter runs for some little distance on the right side of the rectum, and then twists 

 over so as to run in a truly median plane along the posterior face of the rectum almost 

 as far as the anus. 



Along most of its course the right renal caecum is accompanied by the rectal sinus 

 so that the blood thus carried to the heart has some part of its waste matter removed at 

 this stage of its cycle. 



Each nephridium has a single opening (Rn.o.) to the exterior ; it lies at the ventral 

 extremity of each renal tube, and opens into the mantle cavity at the end of a short 

 narrow duct situated close to the parieto-splanchnic ganglion. To ensure more 

 effectively the rapid removal of the renal excretion poured forth by these ducts, each 

 aperture opens into a long open channel or gutter running close to the edge of the 

 adductor. That on tlie right side is particularly well developed ; at first it is narrow and 

 deep, but as it proceeds posteriorly it gradually widens and loses depth until at last it 

 dies away towards the hinder ventral angle of the adductor, the excretion it conducts 

 away being then well under the control of the excurrent outflow from the gills. 



Of the two sections, the anterior region is the most glandular : there the walls are 

 deeply folded and pouched on a somewhat complicated plan (fig 8 - 29 and 30, Neph.r.}. 

 The walls of the internephridial passage and of the posterior csecal region are also fairly 

 extensively folded and puckered to increase the secreting area (fig. 28, I.n.p.). 



The cells lining the walls of all sections of the nephridia are uniform in appearance 

 and size (fig. 27). They consist usually of a single layer of large irregularly cubical 

 secreting cells, very clear and much vacuolated. The cell wall is very distinct, and the 

 nuclei, of large size, are situated towards the bases of the cells. In some places the 

 cells aie somewhat crowded, and occasionally may be two and even three deep. 



The gonad opens in the roof of the internephridial passage by a single small duct 

 (fig. 6, Go.a.}, once more emphasizing the asymmetry of this mollusc. 



