Question

sat practice 8 - october 2, 2025
reading and writing: question 14
the morphological novelty of a bilaterian - marine invertebrates with radial symmetry, usually star - like, around a central point - implies 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 bilateral layout from a known bilateral origin. applying genetic knowledge of xenopus laevis (a common toad, 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, lindsay fishery et al. observed activity only in anterior genes across p. miniatas centre body and some posterior genes limited to the edges, suggesting that
what choice most logically completes the text?
answer
a despite the greater prevalence of anterior genes in sea stars genetic makeup, posterior genes active at the bodys perimeter are primarily responsible for the star - like 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 originated with an atypical body layout that was neither bilaterally nor radially symmetrical
c although the two species are closely related, there is only minimal correspondence in the genetic markers for head, tail, and trunk region development in p. miniata sea stars and x. laevis toad - 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:

The text discusses the morphological oddity of sea - stars (radial symmetry) compared to other animals (bilateral symmetry), and their genetic makeup. Option A makes sense as it ties together the prevalence of certain genes and the sea - star's radial symmetry. It explains how, despite the prevalence of anterior genes, posterior genes active at the body's perimeter are responsible for the star - like form that distinguishes sea - stars' radial symmetry from that of other echinoderms. The other options do not fit as well in terms of connecting the genetic and morphological aspects presented in the text.

Answer:

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 star - like form that distinguishes sea stars' radial symmetry from that of other echinoderms