How chordates and echinoderms separated from each other and the problem of dorso-ventral inversion

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Abstract
It is now possible to reconstruct what happened when the chordates and echinoderms first split from each other. This involves a three-way comparison among: 1) the soluteColeicarpus, which is probably a stem-group dexiothete; 2) the Cincta, which seem to be the least crownward known echinoderms; and 3) the soluteCastericystis, which is a stem-group chordate, probably the least crownward known. Counter-torsion of the tail, by which the effects of dexiothetism were nullified in the tail, took place in two phases, firstly in the fore tail and later in the hind tail. Echinoderms and chordates are descended from ancestors that were attached to, or lay on, the sea floor and were therefore much more liable to attack from above than beneath. This probably explains why the main nerve trunk in chordates is dorsal, rather than being ventral as in protostomes.