Read the following text and answer these questions.

    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. 

    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.

    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.

What is a jellyfish?