Cnidarians. (The sort of animals that includes jellyfish and sea anenomes and coral and such). Theyre so old that the first known predatory animal as far as I'm aware was one of them, and some of them still resemble those ancient versions to a significant degree. Even tho every time theres a mass extinction corals seem to be some of the first things to go, and jellyfish tend to be slow, stupid and not very good at controlling where they go, it somehow works out for them.