The setae vary much in form and are often longer and stronger than in the Oligochaetes.
3) are among the most remarkable forms. Simple bifid setae, such as those of Oligochaetes, are also present in certain forms.
In Polynoe the nephridia are short tubes with a slightly folded funnel whose lumen is intercellular, and this intercellular lumen is characteristic of the Polychaetes as contrasted with leeches and Oligochaetes.
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.