It gets to be about 8-9 inches in diameter. It has a white and green foot, then turns red, then the green nematocysts. They are in a single file line that lead to the center from the outside. It moves extremely fast, inflates and deflates rapidly. Nematocysts move by themselves very quickly. My all black saddle-back clown is now hosting in it too. I woke up this morning to it. Very cool to see the clown thrash inside of it, disappear, then reappear. He likes to just lay still in it. When I first saw him in there, he wasn't moving. It scared the living crap outta me! But then he started thrashing inside of it, so all is well.