Jellyfish don’t have a brain. Instead, the body is made up of a complex nervous system, which is where all of its motor functions and sensory activity occurs. Through the neurons in this system, their body tells the muscles when to contract, which is how they swim. There are over 1000 different species of jellyfish. They are invertebrates that have inhabited the ocean floor for over 500,000 years. Some jellyfish are tiny, some are very large, some are venomous, and some glow in the dark.