Jellyfish have no centralised brain, instead relying on a diffuse nerve net spread across their bodies.
Starfishes have a decentralised nervous system consisting of a nerve ring around their mouth and radial nerves extending into each arm.
Sea urchins have no brain but possess a nerve ring and radial nerves similar to starfish.
Sea sponges are among the most primitive animals, lacking a brain, nervous system, and organs.
Sea anemones are brainless, stationary animals that use a nerve net to detect and react to stimuli like touch and chemicals.
Flatworms lack a true brain, but they have a simple nervous system with two nerve cords running down their bodies and a rudimentary brain-like structure called a ganglion.
Oysters are brainless bivalve mollusks that rely on a simple nerve network to respond to changes in their environment.