This is a beautiful and important group of organisms, the only one on earth that can be seen from orbiting spacecraft. The skeletons of reef-building corals, an important member of the cnidarians, form The Great Barrier Reef which is usually visible from space. Animals in this group are the simplest multicellular animals. That is, there are no animals that are simpler but which are made of lots of cells organised into layers of tissues. Other members of this group include jellyfish, which are famous for their stinging tentacles. The name of the group,in fact, comes from the greek word for "nettle", because the tentacles are loaded with millions of cells that shoot out tiny threads that sting like nettles.

• A body that is made of more than one type of cell (multicellular) organised into layers of tissue.
• Stinging cells (cnidocytes) which can discharge microscopic threads (nematocysts). No other type of animal has cnidocytes.
• To have a body shaped like a like an anemone (polyp) or a like a jellyfish (medusa) (or both at different stages in your life).
• To have a radially symmetrical body.