Fig. 7. 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 1 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 relatively little ⁴⁴Ca had entered the cells and extrathecal coelenteron, with the highest relative concentration being 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 the external seawater layer (SW), oral ectoderm (OE), mesogloea (M), oral gastrodermis (OG), extrathecal coelenteron (ETC), aboral gastrodermis (AG) and calicoblastic ectoderm (CE).