This species constitutes the only true pelagic holothurian (and even echinoderm) known to date. The body of a holothurian is roughly cylindrical. The place of the photograph passes through a fecal mound produced by Molpadia oolitica (holothurian). Walcott's original interpretation as a holothurian was rapidly disputed. "Mackenzia" was originally described by Charles Walcott in 1911 as a holothurian echinoderm.