Authors:Presnell et alAbstract:The current paradigm of gut evolution assumes that non-bilaterian metazoan lineages either lack a gut (Porifera and Placozoa) or have a sac-like gut (Ctenophora and Cnidaria) and that a through-gut originated within Bilateria [ 1–8 ]. An important group for understanding early metazoan evolution is Ctenophora (comb jellies), which diverged very early from the animal stem lineage [ 9–13 ]. The perception that ctenophores possess a sac-like blind gut with only one major opening remains a commonly held misconception [ 4, 5, 7, 14, 15 ]. Despite descriptions of the ctenophore digestive system dating to Agassiz [ 16 ] that identify two openings of the digestive system opposite of the mouth—called “excretory pores” by Chun [ 17 ], referred to as an “anus” by Main [ 18 ], and coined “anal pores” by Hyman [ 19 ]—contradictory reports, particularly prominent in recent literature, posit that waste products are primarily expelled via the mouth [ 4, 5, 7, 14, 19–23 ]. Here we demonstrate that ctenophores possess a unidirectional, functionally tripartite through-gut and provide an updated interpretation for the evolution of the metazoan through-gut. Our results resolve lingering questions regarding the functional anatomy of the ctenophore gut and long-standing misconceptions about waste removal in ctenophores. Moreover, our results present an intriguing evolutionary quandary that stands in stark contrast to the current paradigm of gut evolution: either (1) the through-gut has its origins very early in the metazoan stem lineage or (2) the ctenophore lineage has converged on an arrangement of organs functionally similar to the bilaterian through-gut.
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