FIGURE 2: Transition of the gut mucosal surface in chordates. For animal groups shown in pictograms, intestinal barrier structures are illustrated above. Gut microbes are shown as ovals and circles. Invertebrate outgroups share a peritrophic matrix, although the presence of chitin remains unclear beyond arthropods and annelids. Tunicates possess chitin nanofibers embedded in matrix substances, including gel-forming mucins. Filter-fed microbes are confined to the luminal space. Ray-finned fishes harbor indigenous gut microbes that are separated from the surrounding mucus layer by chitinous barriers. Mammals lost chitin, and the mucus layer is colonized by indigenous microbes. “Yes” or “No” indicates the presence or absence of the item listed on the right, respec-tively. Color-codes are shown in boxes. The bottom tree diagram depicts animal phylogeny, annotated with a series of events that account for gradual changes of the gut mucosa in chordates. We propose a transition from a chitin-based ancestral condition to a mucin-based derived state (top). Adapted from Nakashima et al. Nat Commun 9: 3402.