Explanation
Correct answer: D.
Choice D is the best answer because the passage contrasts sea stars’ radial body plan with the bilateral body plan of their ancestors. Researchers applied body-patterning gene knowledge from closely related acorn worms to sea stars and observed anterior (head) genes active across the sea star’s body, with some posterior (tail) genes active only at the edges. These observations suggest that sea stars’ bodies are largely specified as an anterior region, with posterior identity limited to the edges, rather than being headless. The reported activity pattern supports the choice’s conclusion that sea stars lack a trunk and consist primarily of a head region.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text doesn’t identify how any particular region of sea stars’ bodies influences the layout of sea stars’ radial symmetry. Moreover, the text indicates that the radial symmetry of echinoderms is "usually starlike," not that a starlike layout distinguishes sea stars from other echinoderms.
Choice B
Choice B is incorrect because the text doesn’t suggest that the idea that sea stars evolved from an ancestor with bilateral symmetry is incorrect (describing the bilateral origin as "known") and doesn’t address any body plans other than those with radial or bilateral symmetry. The text strongly suggests that rather than revealing something about sea stars’ origin, Formery et al.’s findings contradict the assumption that the current body plan of sea stars is "headless."
Choice C
Choice C is incorrect because the text suggests that Formery et al. were able to make determinations about P. miniata sea stars’ body plan based on the comparability of genetic markers between P. miniata and S. kowalevskii acorn worms. The text indicates only that little or no activity was observed in certain types of genes associated with body development in P. miniata, not that those genes turned out to largely differ from body-development genes in S. kowalevskii.