Cnidarians are at the tissue level of organization - they lack any structures that can truly be called organs, like a heart or brain. True jellies are only part of the gelatinous zooplankton menagerie, however. All make ends meet without the benefits of a hard skeleton or tough tissue. Lacking dense bodies and hard body parts does have its advantages. Some gelatinous animals, like the sea nettle, Chrysaora fuscescens and the purple-striped jelly, Pelagia colorata , make conspicuous sojourns into nearshore waters on a sporadic basis.
Most types lead far less splashy lifestyles, however, and are overlooked by most people.
Don't count on seeing any of these delicate creatures on your next ocean excursion however. You can go many weeks without seeing a hint of a gelatinous animal. When conditions are appropriate though, nearshore surface waters may be inundated with scores of gelatinous creatures. And then, as quickly as they appeared, the jellies may vanish again into a world of mystery, their whereabouts unknown to those of us confined to breathing air and living on land. Yet even when swarming by the thousands or millions, most folks are totally oblivious to the wonders that may lie just beyond the kelp bed.