In invertebrates, the nerve cord develops ventrally, while in vertebrates, it develops dorsally. The common ancestor of vertebrates and invertebrates appears to have already developed a mechanism for dorsal-ventral polarity, used by all subsequent animals with nerve cords. This is analagous to a shift in the magnetic pole, as though for one phylum, all the compasses started pointing south. Selection for a switch in dorso-ventral polarity would have been easier to make in the larval stage of development, rather than at a later stage. One has to look further back in evolution than insects and worms.
Coelenterates incorporate many of the prototypical design elements found in higher animals.
Although invertebrate sog and vertebrate chordin are homologs, a comparison of their effects on polarity seems to be a paradox. Tolloid has been shown to target Short gastrulation. Tolloid, a putative metalloprotease related to BMP-1, enhances DPP function, while SOG, an ortholog of the Xenopus organizer Chordin, inhibits DPP function. Tolloid is secreted and requires a protelytic processing step for activation. The removal of the N-terminal prodomain is not catalyzed by TLD itself, since it is removed from a putative TLD protease null mutant.