As you can literally see, jellyfish don’t have much for internal organs. Instead of a brain, jellies have what’s known as a neural net — a system of nerve cells interwoven all over the animal’s body. Jellyfish survive this way whether they’re 1 mm wide or 100 feet long.
Not only are they brainless, but sponges don’t even have digestive, nervous or circulatory systems. Instead, they have a bunch of unassigned cells that can go around their body turning into whatever kind of cell is needed at the time. And even without any internal organs, sponges can sneeze! When they detect an irritation, a sponge will “inhale” water and then contract its body and “sneeze” out the irritant. Spongey sneezes can last up to 60 minutes!Source: