Jellyfish are not really fish. They are invertebrate animals. This means that they have no backbones, but in fact they have no bones at all. It also 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 see through them. Some jellyfish can glow in darkness by making their own light by a chemical reaction inside their body. Most of jellyfish live in salt water, but a few live in fresh water. It can found in oceans and seas all over the world. They live in warm, tropical sees, and icy waters near the North and South poles.