Sponges have no nervous system but they DO have a complex network of holes that change function as needed. Cnidaria have the simplest nervous system. Hydra, shown, have the simplest fully mapped nervous system (2017)

Sponges have no nervous system but they DO have a complex network of holes that change function as needed.

Cnidaria have the simplest nervous system.

Hydra, shown, have the simplest fully mapped nervous system (2017)

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1Miso Kochanik