84 



from the latter in texture and also somewhat in colour. 

 It is to a large extent quite independent of the renal 

 organ and has been termed the Nephridial Gland (fig. 49, 

 neph. gl., fig. 50). Seen from the cavity of the 

 renal organ, the nephridial gland differs altogether from 

 the glandular wall of the former. Instead of ridges 

 formed of numerous conical and other filaments, the 

 surface appears as if it were made up of branched and 

 anastomosing fibres, which generally run transversely to 

 the long axis of the gland and renal organ. 



If a solution of methylene green or sauresfuchsin is 

 injected into the foot, one finds after a few hours that 

 the renal organ is taking up the substance from the blood 

 and is deeply coloured by the dye. The nephridial 

 gland, however, is quite free from stain. A second 

 feature of importance is that the nephridial gland is 

 interpolated into the circulation between the renal organ 

 and the auricle, and any injection mass forced into the 

 efferent branchial vessel passes very easily into this gland 

 after filling the auricle. In fact, the nephridial gland 

 is a large spongy blood lacuna. 



The Renal Aperture. — The external opening of the 

 renal organ is situated on the anterior wall separating the 

 gland from the pallial cavity. It lies slightly above and 

 to the left of the rectum. It is a fairly conspicuous 

 opening, slit-like, and of about 3 mm. in length. The 

 long axis of the opening runs dorso-ventrally. The lips 

 of the aperture are thickened owing to the development 

 of muscle fibres, which form a sphincter muscle. The 

 opening leads directly into the lumen of the renal organ, 

 which is only separated from the pallial cavity by the 

 membrane above mentioned. 



The Reno-pericardia! Aperture. — This opening is not 

 situated at the most posterior part of the gland but 



