What class of annelids have protonephridia?

Protonephridia are primarily associated with the class Turbellaria, which includes free-living flatworms, rather than annelids. However, in the context of annelids, the class Oligochaeta (such as earthworms) and the class Hirudinea (leeches) have a different excretory system involving nephridia, which are more complex than protonephridia. In summary, protonephridia are not characteristic of annelids but rather of certain other invertebrate groups.