Nemopilema Nomurai
The Nemopilema Nomurai, a giant jellyfish species, is the l;argest Jellyfish known to man, growing up to 6.6 ft and weighing up to 440 lbs. As you can see, it is not any regular underwater plant, but instead it adjusts from being an immobile Scyphistoma, to becoming a fully developed moving jellyfish. You can also see int he first picture that Jellyfish have an outer and inner nerve ring, but no brain, so they do not exactly have a central nervous system. As far as sensory organs go, they have an epidermis, allowing them to feel their surroundings, along with their basic sense organs, which act as whiskers might on a dog or cat. Also, the jellyfish has a nerve net, which acts as a series of nerves intertwined, linking its sensory organs. Although this amazing creature doesn't have a brain or spinal cord, it still has a complex and efficient nervous system.