Hypothesis of opercular evolution between Serpulae, Crucigera and Hydroides genera. (A) shows funneled Serpula-type operculum, (B) Crucigera-type operculum with basal knobs originating from the bottom of opercular structure, (C) Hydroides-type operculum with defined verticils and basal knobs, (D) advanced Hydroides-type operculum with defined verticils and modified basal funnel originating from basal knobs. Adapted from (Kupriyanova et al, 2008).