Jellyfish have no brain, but they have neurons that seem with the creator's eye-like structure known as rhopalia. They absorb oxygen directly through their skin.
Corals have a simple nervous system known as a nerve net connected through their mouth to tentacles.
Sea stars also called starfish do not have brain. They use seawater to get essential nutrients with the help of the nervous system.
Ascidians do not have brain. These marine animals have dorsal nerve cords, pharyngeal gills, and post-anal tails.
Clams have a nervous system that helps them to feel and react to things instead of brains. They can open and close their shells.
Sea cucumber lacks a central brain but they have a decentralized nerve net that helps them to respond and move to the environment.
These marine animals do not have brains. This plant resemblance animal is master over the bulbous body and frondlike tentacles which help them to feel and hold things.