THE RENAL ORGANS (nEPHRIDIA) OF PATELLA. 373 



dium bad always been above the intestine as in Chiton, and 

 had then passed to the left to take up the position it has in 

 Patella, the channels of communication which attach the 

 kidneys to the pericardium would have both been above the 

 intestine. It may be suggested that the original condition in 

 the ancestors of Patella was similar to that still obtaining in 

 Haliotis and Fissurella where the intestine is surrounded by 

 the ventricle and pericardium. 



The structure of the kidneys in Patella resembles that of 

 other Molluscs. The external orifice leads into a central flat- 

 tened cavity lined throughout by the secreting epithelium 

 which is characteristic of renal organs throughout the Mol- 

 lusca. This central cavity sends off numerous diverticula 

 which branch and ramify and form a spongy tissue beneath 

 the integument on the dorsal surface, and to a less extent in 

 the other regions bordering the kidneys. These glandular 

 diverticula alternate with large irregular lacunae containing 

 blood ; the wall of the diverticulum consists of the secretory 

 epithelium supported by a thin layer of nucleated, fibrous, 

 connective tissue, and from the latter trabeculse of the same 

 tissue pass off here and there and stretch across the blood- 

 lacunse to reach the wall of a neighbouring diverticulum. 



These trabeculse occasionally contain bands of muscular 

 FIBRES. This is true of both the small kidney of the left side, 

 and that of the right, so that both can be contracted to a 

 certain extent. 



The minute structure of the secreting epithelium is some- 

 what difficult to make out ; when a portion of either kidney is 

 teased on a slide in the fresh state and examined in sea water, 

 a number of loose spherical vesicles are seen scattered over the 

 field ; these are of various sizes and contain a varying number 

 of small dark coloured concretions soluble in potash; they are 

 almost all ciliated. When the edge of a piece of renal tissue is 

 looked at similar cells are seen forming a regular ciliated epi- 

 thelium ; when the surface of the same epithelium is observed, 

 the outline of the cells is seen to be polygonal. The great 

 quantity of concretions present makes it impossible to decide 



