In 1822, Geoffroy Saint-Hilaire formulated the hypothesis of a unified body plan throughout the animal kingdom. Being a vertebrate himself, he postulated that all animals would have vertebrae. In vertebrates, the vertebrae are internal, whereas in invertebrates the “vertebrae” were said to be external, as for instance in arthropods where the hypothesized vertebrae are visible as circular cuticular rings. Thus, to Geoffroy Saint-Hilaire arthropods, annelids, and vertebrates could be reduced to the same body plan. However, in contrast to the vertebrates, the dorso-ventral axis of annelids and crustaceans would have had to be inverted such that what is the dorsal surface in deuterostomes is the ventral in protostomes.
The surprising conservation of pattern forming genes between vertebrates and invertebrates has recently revived the discussions of the unity of body plan, including the centuries old idea of dorso-ventral axis inversion (Arendt and Nübler-Jung, 1997; Arendt et al., 2001; Gerhart, 2000).