3) are among the most remarkable forms. Simple bifid setae, such as those of Oligochaetes, are also present in certain forms.
This differentiation is not, however, peculiar to the Polychaetes; for in several Oligochaetes the anterior nephridia are of large size, and opening as they do into the buccal cavity clearly play a different function to those which follow.
In the Oligochaetes it is only the families Aeolosomatidae and Naididae that show the same phenomenon.
Externally, the nephridium opens by a vesicle, as in many Oligochaetes whose lumen is intercellular.