Endostyle-like features of the dorsal epibranchial ridge of an enteropneust and the hypothesis of dorsal-ventral axis inversion in chordates

Classical anatomical investigations of the spengelid enteropneust Schizocar dium brasiliense suggested that the hypobranchial ridge in the ventral midl ine of the pharynx is a homolog of the chordate endostyle. A re-investigati on of pharyngeal anatomy and histology of S. brasiliense does not support t his homology. instead, the dorsal epibranchial ridge of the pharynx of S. b rasiliense provides anatomical and histological correspondences with the ve ntral endostyle of chordates. The potential homology of a dorsal structure in Enteropneusta with a ventral one in Chordata is consistent with a recent evolutionary model for dorsal-ventral axis inversion in the evolution of c hordates. Accepting this homology requires rejecting homology between the e nteropneust stomochord and neurocord (collar cord) and the chordate notocho rd and nerve cord, respectively, but suggests a homology between the entero pneust ventral nerve cord and the chordate neural tube. We propose that fun ctional inversion of the dorsal-ventral body axis occurred in the vertebrat e clade and that the ancestor of the vertebrates lacked dorsal-ventral axis preference, as illustrated by the functional biology of protochordates. Mo reover, body axis shifts may have occurred elsewhere in deuterostomes, part icularly in the evolution of Echinodermata and Ascidiacea.