• Two antennae
  • Four pairs of tentacular cirri
  • Pectinate bars and conical paragnaths on both oral and maxillary rings
  • Parapodia biramous
  • Notosetae homogomph spinigers and falcigers, some falcigers forming simple hooks
  • Neurosetae homogomph and heterogomph spinigers
  • Heterogomph falcigers
Diagnostic description

As the rest of annelids, they exhibit two presegmental regions, the prostomium and peristomium, a segmented trunk and a postsegmental pygidium. Several evidence date the origin of polychaetes in the Cambrian era (around 500 millions before present), but some fossil records of larvae from Pre-Cambrian eras show features resembling polychaete larvae.

The nuchal organs, a pair of chemosensory structures on the postero-lateral margin of the prostomium, are apparently the only synapomorphic trait of the Polychaeta distinguishes them from other Annelida.