Especially are these to be seen in the more modified Oligochaeta and in the much more modified Hirudinea.

This condition is more fully dealt with below in the description of the Oligochaeta.

I. Nephridia.—Excretory organs which are undisputed nephridia are practically universal among the Oligochaeta, Hirudinea and Archiannelida, and occur in many Polychaeta.