Many cnidarian species use alternation of generations to reproduce. In other words, one generation reproduces asexually (the polyps), ususllya by fragmentaion or budding and the next generation reproduces sexually ( the jellyfish). In that way these animals benefit from both types of reproduction
You can read a lot more about jellyfish by clicking on the following link:
https://ocean.si.edu/ocean-life/invertebrates/jellyfish-and-comb-jellies#section_16508
You can also check out the following video:
Watch the video carefully and make a list of all the specific names that come up.