BrcciNUM. 3B5 



THE HEXAL ORGAN. 



The renal organ, the so-called " nephridium," is 

 visible at a glance when the shell is removed. It 

 occupies a position immediately behind the pallial cavity, 

 on the right side of the pericardium above the digestive 

 gland and rectum. The renal organ of Biiccinuvi is not 

 only one of the most highly developed in the Proso- 

 branchiata, but one of the most complicated in the 

 Mollusca. Fundamentally, it is a sac communicating 

 with the pallial cavity and the pericardium. The wall 

 of this sac performs a special function — that of renal 

 excretion, but under no circumstances does the blood 

 system open to the external world in this organ. 

 Externally the sac is covered by the general integument 

 of the body. The outer and inner walls of the renal 

 organ, that is to say, the former underlying the integu- 

 ment and the latter resting upon the digestive gland, are 

 entirely different in appearance. The epithelium of the 

 outer wall is prolonged into filaments or processes of 

 various shapes, which are held together in such a way as 

 to form ridges projecting into and occluding the lumen 

 of the organ (fig. 49). In addition to this epithelium, 

 which is the true glandular layer, there is a supporting 

 connective tissue layer. The other walls are merely a 

 delicate transparent epithelium. The renal organ should 

 be opened by making an incision along the right side 

 close to the digestive gland. The internal surfaces can 

 then be examined. 



It will be noticed, however, without any dissection 

 that another structure is present, lying close to the 

 glandular renal organ proper and forming part of the 

 walls of the sac. This forms a band about | inch wide 

 between the pericardium and the renal tissue. It differs 



