AP BIOLOGY

Detailed explanation-1: -Echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnecting neurons with no central brain, although some do possess ganglia.

Detailed explanation-2: -Echinodermata are marine invertebrates comprising starfish, brittle stars, sea cucumbers, sea urchins, and sea lilies. Animals in this phylum lack any centralized brain and instead possess diffuse neural networks known as nerve nets.

Detailed explanation-3: -Echinoderms possess a unique ambulacral or water vascular system, consisting of a central ring canal and radial canals that extend along each arm. Water circulates through these structures and facilitates gaseous exchange as well as nutrition, predation, and locomotion.

Detailed explanation-4: -Echinoderms have no head or brain. Instead, the nervous system consists of a central ring of nerves with branches extending into each of the arms.

Detailed explanation-5: -Answer and Explanation: The correct answer is (b). Echinoderms do not have gills.