Dorsoventral axis inversion: Enteropneust anatomy links invertebrates to chordates turned upside down

The relationships between chordates with their dorsal nerve cord and other animal groups remain unclear. The hemichordata, specifically the enteropneu sta (acorn worms), have been considered a sister group to the chordata. Ent eropneusts combine Various chordate features (e.g. lateral gill openings, d orsal nerve cord) with features that are usually associated with gastroneur alian invertebrates (e.g. dorsal heart, circumenteric nerve ring, ventral n erve cord). Here we analyse various morphological and functional characteri stics that enteropneusts share with either invertebrates or chordates in th e light of our recent proposal that the chordata may derive - by bodily dor soventral inversion - from a gastroneuralian ancestor. We show that many se emingly non-chordate Features of enteropneusts will align with similar feat ures in the chordates - provided that we compare the ventral side of an ent eropneust to the dorsal side of a chordate. This inversion proposes several interesting and new putative homologies between enteropneusts and acranian chordates, such as between their epibranchial ridge/endostyle (later thyro id gland), their postanal tails, atrial walls, and also between the chordat es' dorsal notochord and the enteropneusts' posteroventral pygochord. Signi ficantly, positional homology between notochord and pygochord is also suppo rted by the expression domains of Brachyury orthologs in vertebrates and in vertebrates: a Brachyury ortholog is active in the posteroventral mesoderm in Drosophila and in the dorsral mesoderm in chordates. In conclusion, we p ropose that the anatomy of enteropneusts may serve as a conceptual 'missing link' between gastroneuralian invertebrates and notoneuralian chordates. W e discuss whether the enteropneust's dorsoanterior nervous centre plus thei r ventral trunk cord then corresponds to brain and dosal nerve cord in the chordata.