Question

the morphological novelty of echinoderms—marine invertebrates with radial symmetry, usually starfish, 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 ______

14 ► mark for review
which choice most logically completes the text?
a despite the greater prevalence of anterior genes in sea stars’ genetic makeup, posterior genes active at the body’s perimeter are primarily responsible for the starfish layout that distinguishes sea stars’ radial symmetry from that of other echinoderms.
b contrary to the belief that they evolved from early ancestors with the bilateral form typical of many other animals, sea stars instead are organized with an atypical body layout that was neither bilaterally nor radially symmetrical.
c although the two species are closely related, there is no direct correspondence in the genetic markers for head, tail, and trunk region development in p. miniata (sea stars) and s. kowalevskii (acorn worms).
d rather than undergoing changes resulting in the eventual elimination of a head region in their radial body plan, as previously assumed, sea stars’ morphology evolved to completely lack a trunk and consist primarily of a head region.

Explanation:

To solve this, we analyze the context: The passage discusses echinoderms (sea stars) with radial symmetry, comparing to other animals. The key is the activity of anterior genes in P. miniata's body. Option B states sea stars evolved from ancestors with bilateral form but have an atypical (radial) layout, which fits: The study applies bilateral - related knowledge to sea stars (radial), and the gene activity suggests their body layout is atypical (not bilateral as expected from ancestors). Option A is about posterior genes vs other echinoderms, but the passage focuses on anterior - posterior axis and gene activity in P. miniata. Option C is about genetic markers correspondence, but the passage's focus is on body layout evolution. Option D is about head region evolution, but the passage says they have a headless layout (evolved from headless, not lost head). So B is correct.

Answer:

B. contrary to the belief that they evolved from early ancestors with the bilateral form typical of many other animals, sea stars instead are organized with an atypical body layout that was neither bilaterally nor radially symmetrical