Fig. 1. Fracture (A) and schematic diagram (B) illustrating the principal features of Ephydatia muelleri: the apical pinacoderm (apd), sub-dermal cavity (sdc), choanocytes (ch), and basal pinacoderm (bpd). The apical pinacoderm consists of an inner layer of endopinacocytes (enp) and and outer layer of exopinacocytes (exp); porocytes (p), which form the ostia (os), are sandwiched between the two layers. The choanosome contains incurrent (in) and excurrent (ex) aquiferous canals, choanocyte chambers (cc) and spicule tracts (sp) that support the apical pinacoderm. A thin collagenous middle region (mesohyl, me) houses mobile cells. Prosopyles (pp), the entrance to chambers are formed by perforate `sieve'-like cells. Apopyles (ap) vent water from chambers. Scale bar, 20 µm.