I think the common jellyfish you see at these zoo and aquarium displays are Aurelia. They can be recognized by the 4 leaf clover leaf design on the top of them. The are carnivores and will also give you a pretty good sting. The Aquarium in New Orleans has an awesome jellyfish display. They have several different tanks. The neatest one that I recall had some that buried in the sand and looked like anemones.

Gary