A group of living things
Jellyfish and corals
Cnidaria
Stinging relatives including reef builders
Its members' last common ancestor lived about 600 million years ago.
You and jellyfish and corals last shared an ancestor about 700 million years ago (on 1 November, if life's history were one year).
Animals in this group share tiny stinging cells to catch food or defend themselves. Each cell holds a coiled thread, often with poison, that fires when touched. That shared sting links corals, anemones and jellyfish as relatives.
What looks like rock can be alive. Most corals are colonies of hundreds to thousands of tiny soft animals related to jellyfish, most no thicker than a coin. Each one catches food with little arms around its mouth, and in reef-building corals each one also builds a hard limestone skeleton. Over many generations those skeletons pile up into a reef, with living animals only on the surface.
The group takes two shapes. Some stay fixed like flowers, with mouth and tentacles facing up, such as corals and anemones. Others swim as bells or umbrellas, with mouth and tentacles facing down, such as jellyfish. Some use both shapes in one life cycle.
Living colour
Many reef corals get their colour and food from tiny algae living inside them, which make food from sunlight.
Named for nettles
Scientists' name for this group comes from an ancient Greek word for nettle, a plant with stinging hairs.
Biggest reef
The Great Barrier Reef is the world's largest coral reef.