I have wondered what a terrestrial ecosystem would look like dominated by cnidarians and ctenophores rather than insects and tetrapods. In order to fulfill the same niches I imagine they would need to be highly derived compared to their ancestral forms. Since their physiology is so different I am having difficulty figuring the path from, say, a jellyfish to a longlimbed Savannah grazer with a mass of tentacles for a face.
Well you need them to evolve some kind of hard tissue to support the body, which could give you animals superficially like echinoderms. specifically sea cucumbers or sand dollars and sea urchins depending on which way you want to point the mouth. But it really depends on how long they have had, the longer they have been evolving for land the less they will look like jellyfish.
If they can somehow separate the hydrogen or the helium from their food or enviroment to keep it in special bladders they could gain buoyancy and float out of the water (where they breed) into the air over dry land. If then they can detect a prey right under them and quickly release gas to drop straight down, they can substitute birds of prey in a ecosystem. Sea animals extract oxygen from the water with gills and such, maybe they can keep the H after taking the O2 (I don't know a lot about underwater respiration so ask someone who does before taking the idea into serious account)
Developing an extensive muscle system throughout their bodies, including their tentacles, could give them mobility--instead of supporting themselves with an endoskeleton or exoskeleton, they could go entirely without skeletons and simply use muscles to hold themselves up.
Perhaps instead of using skeletons or muscles, they could develop hydraulic systems to harden and soften key points in their bodies to allow for locomotion, holding, eating, etc. Perhaps some species could have very minimal, slow moving hydraulics, like plants use to turn towards sunlight, or perhaps some could be as fast as any land animal.
Thicker Outer Skin
To prevent drying out and keep them safe, they could develop a denser, dryer outer layer of tissue, like leather.