Answer: Fish
Step-by-step explanation:
Fishes have central nervous system. The CNS of the fish contains spinal cord, medulla oblongata, and brain.
The brain of the fish is divided into telencephalon, mesencephalon, diencephalon and cerebellum. It is not proven yet that do they feel pain as like animals and humans.
Other organism such as hydra, sponge and flatworm does not contain even brain.