Fig. 2
From: First record of the non-indigenous jellyfish Blackfordia virginica (Mayer, 1910) in the Baltic Sea
The hydromedusa Blackfordia virginica (Mayer, 1910) [14] Leptothecata (Cnidaria: Hydrozoa) cf. Cornelius [15] (a), with hemispherical umbrella of max 22 mm in diameter, varying number of tentacles, depending on development stage, four radial canals and a quadratic manubrium with curved lips. Specific diagnostic characteristics include endodermal cores of tentacles which can project into the bell (grey arrow) (b), closed lithocysts with 1–3 concretions (white arrows) between adjacent tentacles (c), with grey pigmentation at their base (black arrow), even though this characteristic is not diagnostic for all B. virginica sub-populations (d), all images from living material