Fig. 8. Low spatial resolution SIMS images of a 2 µm-thick, freeze-substituted section of Galaxea fascicularis showing the distribution of (A) ⁴⁰Ca and (B) ⁴⁴Ca after 8 min incubation in ⁴⁴Ca artificial seawater in the light. The ⁴⁴Ca/⁴⁰Ca ratio image (C) and line profile (E) measured over A–B in (C) indicate that ⁴⁴Ca has reached a higher relative concentration in the mesogloea and extrathecal coelenteron than in the external seawater layer. Grey values in E represent pixel intensity values along A–B in (C) as a function of distance (µm). (D) Adjacent section, stained with Toluidine Blue, showing external seawater layer (SW), oral ectoderm (OE), mesogloea (M), oral gastrodermis (OG), extrathecal coelenteron (ETC), epithelia (AE) and skeleton (SK).