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ImageThey 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.


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.


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.


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.


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.


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.


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.


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 attract animals they prey on.

Source: Microsoft ® Encarta