Sponge phylogeny and the evolution of epithelial characters.

A phylogeny of Metazoa adapted from Philippe et al 2009 which considers Porifera monophyletic. Sponge phylogeny is adapted from [48]. Freshwater sponges belong to the group indicated with a box (G4). Letters indicate evidence of epithelial characters: (a) transepithelial resistance (TER) and transepithelial potential (TEP) in freshwater sponges; (b) septate junctions in calcareous sponges; (c) a basement membrane in homoscleromorphs; (d) absence (loss?) of a basement membrane in placozoans; (e) true epithelia with septate junctions, a basal lamina, TER and TEP in cnidarians and bilaterians. Arrows indicate three potential origins of epithelia; the solid bar indicates the most parsimonious scenario for the origin of epithelia.