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.
How would you define oligochaetes? Add your definition here.