Fig. 3. STEM images of 1 µm-thick, freeze-substituted sections of Galaxea fascicularis showing typical regions of the oral epithelia. (A) The oral ectoderm (OE), containing dense cnidocytes (cn), is separated from the oral gastrodermis (OG) by the mesogloea (m). Two zooxanthellae (z) are present in the OG, and a part of the extrathecal coelenteron (c) is also shown. (B) The OE and OG are shown, containing numerous dense mucocytes (mu). Part of the extrathecal coelenteron is also shown. Elemental images of Na, Cl, P, S, K and Ca are shown, with the concentration of each element represented by a thermal colour scale (in mmol kg^–1). The Cl and Na images indicate the presence of NaCl in the extrathecal coelenteron and mesogloea.