Jellyfish have no gills or lungs. They have no hearts or brains. In fact, jellyfish have no bones or skeletons to give them shape or to support their bodies. The jelly does that.

A jellyfish’s body looks a lot like a bell or an umbrella. Scientists call this body style a medusa (muh DOO suh). A jellyfish’s mouth is a small opening on the underside of the bell. It is the only opening leading into the jellyfish and the only opening leading out.

A jellyfish has many tentacles. Some hang around the bell of the jellyfish. These hold the stinging cells. Other tentacles hang around the mouth. These are called oral arms. They may or may not have stinging cells. The oral arms pass the food into the jellyfish’s mouth. The food then goes into the animal’s stomach.

If you are thinking of the jelly you eat with peanut butter, the answer is no. That jelly is fruit and fruit juice mixed with a thickener.

But jellyfish are made of another kind of jelly. Jellyfish jelly is an amazing blend of salt, protein, and lots of water. In fact, most jellyfish are over 95 percent water.

A jellyfish has a very simple body. Two layers of cells make up the body walls. A thick layer of jellyfish jelly between these walls helps give the jellyfish its shape.

Jelly is a perfect material for the body of an animal that lives in water. It helps support the jellyfish in the water. It helps jellyfish that live near the surface to float. And it keeps deep-sea jellyfish from being crushed by the pressure of the water.

Humans learn about their world by using their five senses. But most jellyfish don’t have eyes, ears, a nose, a tongue, or hands. These jellyfish depend on simple sense cells. The jellyfish’s sense cells are located in ridges along the edge of its bell.

These sense cells perform different jobs. One kind acts as eyes and senses light. Sensing light helps the jellyfish know if it is upside down in the water. Another kind acts as a nose and senses chemicals in the water. A third kind helps a jellyfish keep its balance in the water.

Some jellyfish, such as sea nettles, make their own light. They glow or give off flashes of light as fireflies do. Some jellyfish use this light to attract prey. But most jellyfish use it as a defense against predators.How do its lights help a jellyfish? A jellyfish may light up to surprise a predator or to frighten it away. Lighted up, a small jellyfish with long tentacles suddenly looks like a large animal.

One jellyfish drops its glowing tentacles when fleeing from a predator. The attacking animal swims after the falling tentacles. This gives the jellyfish time to get away.

Jellyfish may light up for other reasons, too. To find out what those reasons are, scientists dive down in ships called submersibles (sub MER suh buhlz). The ships have windows that let scientists study the jellyfish deep in the ocean where they live.

Young jellyfish look very different from their parents. In fact, a young jellyfish goes through several stages before it becomes an adult. Other animals go through life cycles, too. Think about the way in which a caterpillar becomes a butterfly or a tadpole becomes a frog.

Most large jellyfish start out as eggs. A jellyfish egg develops into a larva (LAR vuh). The larva drifts along in the ocean currents until it settles and attaches itself to the ocean bottom.

Next, the larva grows into a polyp (PAHL ihp). As the polyp develops, its stem begins to look like a stack of tiny saucers. One by one, the “saucers” break off, and each one develops into a tiny medusa. In this way, a single larva produces many jellyfish.

Many jellyfish that live deep in the ocean are red—like this dunce cap jellyfish. Others are purple, brown, or black. These colors help jellyfish in two ways. They help hide what a jellyfish has eaten. And they make it hard for predators to see the jellyfish.

In the deep ocean, being able to hide a meal can be very important. The reason is that many deep-sea animals use light—much the same way jellyfish do. Imagine what would happen if a clear jellyfish ate a glowing fish. That’s right! The fish would show through the jellyfish’s bell. But a colored bell hides the meal, so predators cannot see the fish.

Very little sunlight reaches deep down into the ocean. A clear jellyfish would reflect this light, almost the same way a mirror would. But dark colors do not reflect light. The dark colors of the jellyfish help camouflage (KAM uh flahzh), or hide, it from its predators.

The lion’s mane is the largest kind of jellyfish. Its bell can grow to widths of more than 7 feet (2.1 meters). And its long tentacles can extend more than 130 feet (40 meters). This jellyfish can be found in the cold waters of the Atlantic and Pacific oceans. The largest lion’s manes are found in the Arctic Ocean.

The lion’s mane jellyfish is named for its shaggy oral arms and hairlike tentacles. Its stings are toxic, or poisonous. To catch prey, this jellyfish sinks down in the water. It spreads its tentacles out around the prey. The tentacles act like a huge net to catch the prey.

Most jellyfish swim through the water with their tentacles and oral arms hanging below them. But not Cassiopeia (kas ee uh PEE uh), the upside-down jellyfish. This stinger spends most of its life with its tentacles floating above it.

Cassiopeia is a bottom-feeder. It lives in the shallow water of swamps near the seacoast. It sinks under the water and uses its bell as a suction cup to hold onto the bottom. Then Cassiopeia waves its oral arms to catch passing zooplankton.