A growth both of the funnel, which becomes multicellular, and of the rest of the nephridium produces the adult nephridia of the genera mentioned.
I, The sub-anal tract of the large nephridium given off near its papilla and seen through the unshaded smaller nephridium.
C, Small portion of the nephridium of Glycera siphonostoma, showing the canal cut through, and the solenocytes on the outer surface.
There are thus substantial reasons for believing that the nephridium grows backwards from a funnel as does the coelomoduct.
Eg, Stem of the nephridium leading to no, its external aperture.
In the male there are a right and a left protrusible penis in every genital segment, into which opens the nephridium and a sperm-sac. The wide funnels of the nephridia of this region are possibly of coelomic origin.
The Polychaeta, however, present us with another form of nephridium seen, for example, in Arenicola, where a large funnel leads into a short and wide excretory tube whose lumen is intercellular.
R, Nephridium (kidney).
It is further noticeable that in Rhynchelmis the covering of vesicular cells which clothes the drainpipe cells of the adult nephridium is cut off from the nephridial cells themselves and is not a peritoneal layer surrounding the nephridium.
Vezhdovsky's figures of Rhynchelmis agree with those of Bergh in showing the backward growth of the nephridium from the funnel cell.