Jelly fishes do not have brain and spines. You can also see right through them. But they can cause painful, sometimes deadly stings. Some can even glow in the dark. Scientists know them as medusas, however some people call them as jellyfish. What are jelly fish? They are not really fish. They are invertebrate animals. This means they don’t have backbones. In fact, they don’t have bones at all.