The portion of Carniola belonging to the Karst region presents a great number of caves, subterranean streams, funnels and similar phenomena.
In Lumbricus the connexion is a little closer; the funnel of the nephridium, in the segments in which the funnels of the gonad ducts are to be developed, persists and is continuous with the gonad duct funnels on their first appearance.
At the least there is no doubt that the genital funnels are developed precisely where the nephridial funnels formerly existed.
If the genital funnels are not wholly or partly formed out of the nephridial funnels they have replaced them.
Here also the genital funnels have either replaced or been formed out of nephridial funnels.