Jellyfish.

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Jellyfish.
They have no brains and no spines. You can often see right through them. But they can cause
painful, sometimes deadly stings. Some can even glow in the dark! Scientists know them as
medusas, but most people call them jellyfish.

WHAT ARE JELLYFISH?
Jellyfish are not really fish. They are invertebrate animals. This means that unlike fish or people, they
have no backbones. In fact, they have no bones at all.
Jellyfish have stomachs and mouths, but no heads. They have nervous systems for sensing the world
around them, but no brains. They are made almost entirely of water, which is why you can look through
them.
Most jellyfish live in salt water, apart from a few types that live in fresh water. Jellyfish are found in
oceans and seas all over the world. They live in warm, tropical seas and in icy waters near the North and
South poles.

BELL-SHAPED BODIES
Adult jellyfish are shaped like an open umbrella or a bell. Tentacles dangle down from the edges of a
jellyfish's body. At the center of a jellyfish's body, where it is widest, is the mouth.
The ends of the tentacles have stinging cells on them, which jellyfish use to paralyze and catch their
prey. The sting from a jellyfish can be very painful. Some jellyfish, such as the sea wasp, have powerful
enough stings to kill people.

WHAT DO JELLYFISH EAT?
Most jellyfish eat fish, tiny creatures called plankton, and even other jellyfish. Once a jellyfish stings and
paralyzes its prey, it takes the prey to its mouth and swallows it.

MANY SIZES AND COLORS
Some jellyfish are very small, no larger than a marble. Others are quite large. The largest jellyfish can
grow to about 7 feet (about 2 meters) across and weigh hundreds of pounds! Jellyfish may be blue,
orange, pink, yellow, purple, and other colors.

HOW DO JELLYFISH MOVE?
Jellyfish use their bell-shaped bodies to move themselves through the water. They expand their bodies
and then squish their sides together. This pushes water out behind them, propelling them along. Most
jellyfish swim very slowly. They do most of their moving by riding on ocean currents.

THE LIFE OF A JELLYFISH
Life for a jellyfish begins as an egg. Adult jellyfish release thousands of eggs into the water. The eggs
develop into larvae. These larvae attach to rocks or other objects on the ocean floor and grow into a
polyp. Polyps usually grow in colonies, or groups. They look like plants because they attach to the
ground and grow upwards.
The polyps grow taller until they look like tall stacks of saucers. The saucers gradually detach
themselves from the polyps and swim free. These free-swimming saucers are young jellyfish.
The jellyfish then grow into adults, lay eggs, and the life cycle begins anew.

JELLYFISH THAT GLOW
Some jellyfish can glow in darkness by making their own light. The light is made by a chemical reaction
inside the jellyfish. Scientists believe jellyfish glow for several reasons. For example, they may glow to
scare away predators or to attract animals they like to eat.
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