The moon jelly room is part of the Monterey Bay Aquarium's "Jellies: Living Art" exhibit which opened last month. Organized in terms of three themes — shape and size, rhythm and movement, color and pattern — the exhibit lets visitors see that jellies are not merely amorphous stingers. The world's 200 or so species of jellyfish are diverse and beautiful, like living, breathing sculptures.

Some jellies look as delicate as tiny glass ornaments with lines of silver painted inside them — such as the umbrella jelly, which is about the size of a penny. Others look like red and orange explosions — such as the ocean's biggest jelly, the lion's mane. In Arctic waters, this spectacular creature can boast tentacles longer than a football field and a body that can reach up to eight feet in diameter (that's roughly the width of a school bus). And then there's the surprising flower hat jelly, which looks like an ornate, beaded hat suspended in the blue.

• We've put together a pop-up jelly gallery for you to see more amazing jelly pictures.

Invisible Enemies
The ghostly, billowing movement of these creatures seems at odds with the painful, hard reality of their stings. As many unsuspecting swimmers have found, jelly venom ranges in potency, from irritating, rash-causing stings to those that can cause fatal complications.

One of the deadliest jellies in the world, the Irukandji jelly, is virtually invisible and is not much bigger than a peanut. Last month, American tourist Robert King died after being stung by an Irukandji while he was diving on the Great Barrier Reef off the northeast coast of Australia. King was the second recorded fatality by an Irukandji sting: the first was British tourist Richard Jordan, who died in January. Irukandji jellies are usually found offshore, but this year unusually high numbers of them have been washed up on north-Australian beaches because of prevailing wind conditions.

Irukandji venom causes distressing symptoms that include intense cramping, vomiting, and a sense of impending doom, where victims feel that something terrible is going to happen to them. Together, these symptoms are known as "Irukandji syndrome." In Robert King's case, the venom caused extraordinarily high blood pressure which lead to a fatal stroke.

Jelly venom is discharged through stinging cells called "nematocysts," which are found in high concentrations along the tentacles. The venom injected by the stinging cells makes its way around the human body through our built-in transport system, the circulatory system. As the blood carries venom around the body, the venom affects the brain, lungs, stomach and intestines, and muscles, causing a variety of problems in the victim.

• You can learn how the human circulatory system works using an activity from Middle School Science Gateways, The Human Circulatory System. To use it, you need to be a subscriber to Riverdeep.net. Or you can get a free-trial subscription for 30 days. You'll also need the Logal Express plug-in.

Sea Wasps
Species of box jellies or "sea wasps" are thought to be responsible for about 65 deaths over the past 50 years in Australia. Most people who get stung by box jellies do survive, but they arrive at hospitals in miserable condition and can take some time to recover.

Dr. Jamie Seymour of James Cook University has spent many years studying the box jelly. He explains that when a person gets stung, their reaction can make the sting much worse: "When the tentacles come in contact with you, they stick to you like superglue. People get stung in the legs, then they put their hands down to see what's wrong, then they get it on their hands, then on their chest... You end up with somebody who gets totally wrapped in tentacles." The box jelly's tentacles can be up to 10 feet long.

From November to May of each year is "stinger season" in northern Australia, when stinger nets are placed in the water along many beaches to keep box jellies away from the shallow areas where people bathe. "We have miles and miles of golden sandy beaches, but in the stinger season everybody's crammed into a 100 yard by 100 yard area enclosed by stinger nets," says Dr. Seymour. "If you swim outside the nets, it's not a matter of if you get stung, but when you get stung. For the tourist industry it's a huge problem. We've just had these two deaths, so it's become a major issue."

The work Dr. Seymour and his students are doing should help protect swimmers from box jellies. They've developed a computer model that accurately predicts the beginning and end of the stinger season, so stinger nets can be put up and removed at the right times — and so swimmers will know when to take extra precautions (such as wearing body suits).

Jellies don't seek out humans to sting — they'd much rather keep to themselves. Stinging is their only defense mechanism when they accidentally come into contact with people. So why do box jellies have such powerful venom? "People have suggested that they need venom to protect themselves because they're so fragile and might break up. That's not the case at all — they're as robust as a football," says Dr. Seymour. Nor does the venom protect box jellies from predators; their main enemy, the turtle, suffers no ill effects when it gobbles jellies up.

Box jellies are hunters, and their venom is the hunter's weapon, used to paralyze and kill fish for them to feed on. Because of the similarity between the fish nervous system and the human nervous system, the jelly's venom affects us just as it affects the fish it was designed to kill.

Jellies at Home
Swimmers along the United States' coasts need not fear jellies as deadly as the box jelly and Irukandji jelly. If you're swimming in the Atlantic or Pacific Ocean this summer, you're more likely to bump into a drifting moon jelly, whose sting causes a temporary, itchy rash. Moon jellies are widely distributed and can be found bumbling along the length of both coastlines.

Swimmers on the east coast are familiar with East Coast sea nettles, named for their painful sting which some say is similar to a bee sting. (This jelly's Pacific counterpart is known as the West Coast sea nettle.) The Portuguese man-of-war, found in the northern Atlantic, is a more notorious jelly whose sting is very painful and can cause a severe reaction. Fortunately, encounters with it are rare.

Above all, remember that jellies aren't looking for trouble; you're unlikely to get chased out to sea by a rogue jelly with criminal intent. If you see a jelly drifting towards you, your best bet is to swim the other way and carry on enjoying your day at the beach. (If you do get stung, there's some advice on what to do above right.)

• You can visit the Monterey Bay Aquarium's jelly exhibit — "Jellies: Living Art" — online.
• Meet many jellies at the Tennessee Aquarium.
• What are jellies made of? Do they have blood? Do they have muscles? How do they know which way is up? How do they breathe? Find the answers to these questions in the National Aquarium in Baltimore's excellent jelly pages. Also included are some fun jelly facts.