Is the evolution of corals, sea anenomes and jellyfish intrinsically linked to the point where if say corals did not evolve, neither would jellyfish? Or vice versa.
I found this article with phylogenetic trees of Cnidaria answering your question. You should note that phylogenetic tress are statistical models based on sequencing. They may change depending on the methods used, the organismed sampled and the genes used for comparison I translated the groups you mentioned to the following biological names: Corals - Hexocorallia and Octocorallia Sea anenomes - Actinaria (Hexocorallia) Jellyfish - Medusozoa (Not the same levels)
Judging by Figure 3 (don't know if I'm allowed to post the figure to this site) And my limited knowledge in the subjet they each group could've evolve seperatley with a higher linkage between jellyfish & corals(octocorallia) than to sea anenomas. Sea anenomas and corals(hexocorallia) are very closly linked and sea anenomas appear at the first junction of the tree which means that if sea anenomas did not exists neither the rest of the group.