What comprises the echinoderm nervous system?

• A. Brain and spinal cord
• B. Nerve ring and radial nerves
• C. Diffuse nerve net
• D. Ganglia only

Answer: (B)

Echinoderms organize their nervous system around a circumoral nerve ring with radial nerves extending into each arm. This ring coordinates signals and movement across the body without a centralized brain or spinal cord, fitting their radial body plan. While some neural tissue can be diffuse, the defining feature is the nerve ring plus radial nerves, not a brain-and-spinal-cord arrangement or solely lanes of diffuse nerve net. The presence of ring-like coordination and extending nerves best describes how their nervous system is built.