On adults, the translucent whitish cnidosacs (1) inside the distal tips of cerata are mostly concealed by a subapical, opaque white band (2) which is narrow at the posterior and shield-shape at the anterior. The shield extends a short distance down the anterior of the cerata, but there are no white pigment splashes on the rest of the ceratal surface.
F. bostoniensis defends itself by the release of stinging cnidocytes sequestered from its prey and stored in the cnidosacs.
Outer Hebrides, Scotland. April 2018. Leg. D. McKay & S. Taylor.