Jellyfish are from the phylum Cnidaria. This phylum contains over 9,000 aquatic species. This phylum is divided into four definitive classes which include all anemones, corals, fire corals and what is commonly referred to as true jellyfish. Anthozoa contains sea coral and anemones. Hydrozoa includes the Portuguese Man o`War which contrary to popular belief is not an actual jellyfish but a massive colony of hydrozoans. The class Cubozoa contains box jellies, the deadliest creatures on Earth. True jellyfish belong to the class Scyphozoa which includes over 200 species. Notice starfish are not a member of this phylum.
Starfish or sea stars are echinoderms belonging to the class Asteroidea. They are members of the Phylum Echinodermata. So what do starfish have in common with jellyfish? They are both truly aquatic invertebrates, meaning they can not survive for extended periods of time without being submerged in water and neither has a backbone, an essential part of a higher lifeforms advanced skeletal system.
Frogs are amphibious, They can survive in either water or open air environments. They are vertebrates, having both a backbone and an advanced skeletal system. Therefore they are not as closely related on the evolutionary scale as jellyfish and starfish.
Jellyfish are actually quite interesting. Just thirty years ago scientists thought it was impossible to keep them alive in captivity. Ten years ago there was no such thing as a Jellyfish aquarium fish tank. Now you can keep pet jellyfish in a home aquarium.