BUCCINUM. 337 



laterally, on the inner wall of the pericardium, and some 

 distance away from the main cavity of the renal organ. 

 The opening is much smaller than the external renal 

 aperture and often difficult to find, though it is rendered 

 more conspicuous by the somewhat white lips standing 

 out on the darker background of membrane. It is not 

 easy to see what may be the use of the reno-pericardial 

 canal and opening. In those molluscs in which a 

 pericardial gland is present as an accessory excretory 

 organ, the products, of course, would pass to the exterior 

 by this canal. Pericardial glands are, however, not 

 widely distributed in the gastropoda, and appear entirely 

 absent in the whelk. 



Structure of the Renal Organ. 



In considering the vascular system of the whelk, 

 reference has already been made to the large sinus which 

 lies under the floor of the renal organ. From this sinus 

 a number of vessels arise, which, after crossing the lumen 

 of the nephridium, give rise to branches which pass to 

 tlie filaments of the ridges (fig. 49, ren. r.). Each fila- 

 ment is an evagination of renal epithelium, and each 

 contains a blood cavity. The cells of this epithelium are 

 of two kinds, glandular and ciliated, but the former are 

 by far the most numerous. The ciliated cells occur on the 

 summits of the filaments (fig. 52, reii. cil.) and pass 

 gradually into the gland cells. The gland cells are 

 regular columnar cells, the height not greatly exceeding 

 the width. The cytoplasm is very regularly vacuolated, 

 and remains as a kind of fine network in the cells, staining 

 reddish violet with methyl-blue-eosin. Very large 

 vacuoles are not usually present. The nucleus is situated 

 at the base of the cell. Underlying both kinds of cells is 

 the continuous supporting membrane of connective tissue 



