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The morphological novelty of echinoderms—marine invertebrates with radial symmetry, usually starlike, around a central point—impedes comparisons with most other animals, in which bilateral symmetry on an anterior-posterior (head to tail) axis through a trunk is typical. Particularly puzzling are sea stars, thought to have evolved a headless layout from a known bilateral origin. Applying genomic knowledge of Saccoglossus kowalevskii acorn worms (close relatives of sea stars, and thus expected to have similar markers for corresponding anatomical regions) to the body patterning genes of Patiria miniata sea stars, Laurent Formery et al. observed activity only in anterior genes across P. miniata’s entire body and some posterior genes limited to the edges, suggesting that ______blank
Which choice most logically completes the text?