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. 2015 Dec 19;370(1684):20150048. doi: 10.1098/rstb.2015.0048

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© 2015 The Author(s)

Published by the Royal Society. All rights reserved.

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Figure 4. — Three schemes for evolution of the chordate and hemichordate nerve cords. (a) The ancestor of chordates and ambulacrarians had a nerve net. Nerve cords in hemichordates and chordates evolved independently [54]. (b) The ancestor of the Ambulacraria plus Chordata clade had a ventral nerve cord. A dorsal/ventral inversion occurred at the base of the ambulacraria plus Chordata. Thus, the dorsal nerve cord of hemichordates is homologous to the chordate nerve cord. (c) The ancestor of the Ambulacraria plus Chordata had a ventral nerve cord. A dorsal/ventral inversion occurred in the lineage leading to chordates. Thus, the ventral nerve cord of hemichordates is homologous to the dorsal nerve cord of chordates. Adapted from [55].