Question

Which trait evolved in a common ancestor of cnidarians, flatworms, mollusks, and echinoderms?

a) Bilateral symmetry
b) Radial symmetry
c) Segmentation
d) Exoskeleton

Answer 1

Common ancestors of cnidarians, flatworms, mollusks, and echinoderms evolved the trait of bilateral symmetry, despite echinoderms later exhibiting radial symmetry as adults.

Step-by-step explanation:

The trait that evolved in a common ancestor of cnidarians, flatworms, mollusks, and echinoderms is bilateral symmetry. Bilateral symmetry involves division through a midsagittal plane, creating two mirror images, which is a characteristic seen in the Bilateria clade. Even though adult echinoderms exhibit radial symmetry, their larval forms exhibit bilateral symmetry, indicating their evolutionary roots in bilaterally symmetrical ancestors.