018 R. J. HARVEY GIBSON ON THE 



most other parts. There are, however, points in their anatomy and histology 

 which do not appear to have received sufficient attention. 



The left nephridium is by far the smaller of the two, and occupies the 

 triangular space between the pericardium and the terminal portion of the 

 rectum. It is bounded above by the visceral integument, and beneath by the 

 dorsal wall of the subanal portion of the left nephridium; to the right it is 

 bounded by the wall of the rectum. 



The left nephridium communicates with the pericardium by a minute canal. 

 On laying open the pericardium, the opening of this canal may, in specially 

 large limpets, be seen (PI. CLI. fig. 36). The one from which the figure is 

 taken was about 2^ inches long with the shell removed. A split bristle was 

 inserted into the aperture, and on dissection was found to have penetrated into 

 the cavity of the left nephridium. The pericardial opening was situated just 

 beneath the attachment of the auricle, and very near the front wall of the 

 pericardium. It lay almost in front of the much larger opening of the right 

 nephridium. The left nephridium communicates also with the exterior by a 

 papilla which lies to the left of the anal papilla. The left nephridium itself 

 consists of a sac whose walls are folded and ridged to an enormous extent, so 

 that the central cavity is broken up into a series of diverticula. The central 

 cavity remains continuous with the duct into the pericardium on one side and 

 to the exterior on the other. The canal leading from the pericardium to the 

 left kidney is lined by squamous epithelium continuous with the epithelium 

 lining the pericardium. Towards the cavity of the kidney itself the squamous 

 epithelium becomes cubical and then ciliated, and contains granular concretions. 

 The cilia are exceedingly difficult to preserve, and only a cell here and there 

 showed the cilia at all satisfactorily. 



The author has not been able to see the " triangular piece of tissue," de- 

 scribed by Cunningham as functioning as a valve at the opening into the kidney, 

 but his sections may not have been in this respect so favourable. The canal is 

 surrounded by a quantity of connective tissue and nonstriped muscle, by the 

 contraction of which it may be possible to occlude the canal altogether. The 

 valve under such circumstances seems to be rather superfluous, though the 

 writer is not prepared to say it does not exist. The folded wall of the kidney 

 is composed of connective tissue containing a large quantity of nonstriped 

 muscular fibre. In the connective tissue there are a large number of lacunar 

 blood spaces ; in short, as well put by Lankesteu, " the sac is practically a 

 series of blood-vessels covered by renal epithelium." The renal epithelium is 

 in some points difficult to make out in its structure. 



It has been described as consisting of ciliated nucleated columnar cells 

 containing small dark-coloured concretions. 



So far as the author has been able to make out from a study of the 



