Incertae sedis Pleistoannelida is a group of annelid worms that comprises the vast majority of the diversity in phylum Annelida.

[6][5] New findings have discovered the range of Annelid diversity have led to uncertainty if groups with developed ancestral traits should remain within the clade.

A possible, closer sister group to Pleistoannelida could be Dinophiliformia, a clade containing interstitial (meiofaunal) genera previously found in Orbiniida (Sedentaria).

[8] Palaeoannelida Chaetopterida Amphinomida Lobatocerebrum Sipunculida Dinophiliformia Eunicida Phyllodocida Protodriliformia Myzostomida Spinther Aberranta Orbiniida Cirratuliformia Siboglinidae Sabellida Spionida Opheliida Echiura Capitellidae Terebelliformia Maldanomorpha Clitellata A 2019 analysis recovered the mesozoan group Orthonectida as the sister group to Pleistoannelida.

[11] Besides the eyes, it has also been studied that commissures in the brain, glanglia, and nuchal organs were to have also evolved in the lineage of Pleistonannelida (Errantia and Sedentaria) by observing Magelona mirabilis.