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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.