a Line drawing, and b, c thin sections of fully developed popcorn-shaped cassiosomes from semithin sections (~1 µm) of resin-embedded vesicular appendage (see Fig. 8). Cassiosome peripheral layer comprising nematocytes (cyan arrows) bearing O-isorhiza nematocysts (as peripheral dark spheres stained blue with Richardson’s stain in (b) and (c)) interspersed with patches of oddly shaped ectoderm cells (red arrows), and motile cilia (pink arrows); blue-stained nuclei (yellow arrows) visible below the base of large nematocysts capsule in nematocytes, and also in non-nematocyte ectoderm cells. Cassiosome core containing presumptive mesoglea (gray central region in (a), gray arrow in (b and c)), speckled with amoebocytes (purple arrows)—hosting Symbiodinium (green arrows) or empty. Rigid stereocillia/cnidocil complex (orange arrows) visible as a point at the nematocyst apical portion, and deployed tubules (black arrows in (a)) on surface present as long, thick spiny threads. epi vap = epithelial layer of the vesicular appendage pocket. Scale bars = 50 µm.