From Sikorski e al (2918: 963): "the prostomium and peristomium are completely un-fused; branchiae are present along nearly all segments; often capillaries are arranged in more than two vertical rows in anterior chaetigers, and often transversal dorsal membranous crests present in the ‘post nuchal organ’ area."
Sikorski, Andrey; Gunton, Laetitia M.; Pavlova, Lyudmila. (2017). Laonice species (Polychaeta, Spionidae) from the Whittard Canyon (NE Atlantic) with descriptions of two new species. Journal of the Marine Biological Association of the United Kingdom. 97(05): 961-973.