(A) The transition from single-cell organisms to complex multicellular animals was enabled by an extracellular matrix. (B) Electron microscopy (EM) and immunohistochemistry (IHC) of the Ctenophora species, Mnemiopsis (IHC: 20X magnification), Pleurobrachia (IHC: 20X magnification), and Beroe (IHC: 40X magnification) and ECM components of Ctenophora. (C) Electron microscopy (EM) and immunohistochemistry (IHC) of the non-bilaterian animal phyla, Cnidaria (Nematostella; 20X magnification), Placozoa (Trichoplax), and Porifera (Homoscleromorpha and Demosponges) and ECM components of Porifera, Placozoa, and Cnidaria. Demosponge EM reproduced from Figure 1E of Adams, et al., Freshwater Sponges Have Functional, Sealing Epithelia with High Transepithelial Resistance and Negative Transepithelial Potential, PLoS ONE, 2010, volume 5; Homoscleromorph EM reproduced from Figure 3B, Leys et al., Epithelia and integration in sponges, Integrative and Comparative Biology, 2009, volume 49 with permission from Oxford University Press; Homoscleromorph IHC reproduced from Boute et al., Type IV collagen in sponges, the missing link in basement membrane ubiquity, Biology of the Cell, 1996, volume 88 with permission from Wiley; Trichoplax EM reproduced from Ruthmann et al., The ventral epithelium of Trichoplax adhaerens (Placozoa): Cytoskeletal structures, cell contacts and endocytosis, Zoomorphology, 1986, volume 106 with permission from Springer. (D) ECM components in choanoflagellates, the unicellular sister-group to metazoa. All scale bars 500 nm, unless otherwise noted.