Echinoderms are descended from bilaterians, but many members of the group lack a brain, have no specialized sense organs, and have radial nerves that extend down each "arm" from a central nerve ring that surrounds the gut. What was a likely selective pressure in the evolution of this type of nervous system?

Question

Quizplus · Accepted Answer

Echinoderms, such as starfish, exhibit a unique form of radial symmetry as adults, which is a departure from the bilateral symmetry seen in their larval stage and in their bilaterian ancestors. This radial symmetry, along with the lack of centralized brain structures and specialized sense organs, suggests that the evolution of their nervous system was likely influenced by their sessile or slow-moving lifestyle and the need for sensing and interacting with the environment in all directions equally, rather than by the factors listed in the other options. None of the provided options directly addresses the selective pressure for the development of a radial nervous system that extends from a central nerve ring, which is more related to their mode of interaction with the environment and less with forward movement, predatory role, multicellularity, or having an endoskeleton of calcium carbonate.

Echinoderms Are Descended from Bilaterians, but Many Members of the Group