There are many interesting creatures in the phylum cnidarians but the Portuguese Man-Of-War is one of the deadliest (Box Jellyfish or "Sea Wasps" kills the most people). They form floating colonies that contain several specialized kinds of polyps. In each Portuguese Man-Of-War, one polyp forms a balloon like float that keeps the colony on the surface. Another dangerous jellyfish is a tiny Australian jellyfish that has enough toxin to kill within 3-20 minutes.
Some jellyfish for example, Lion’s Mane, are found in the North Atlantic and usually grow up to 2 metres in diameter but the largest ever found was 3.6 metres in diameter with tentacles 30 metres long (that's about a third of a football field!).
Heart's Reef in Australia's Great Barrier Reef
Sea anemones can grow up to a metre in diameter and along with many other cnidarians, they can create magnificent coral reefs. The Great Barrier Reef off the coast of Australia is 2000 kilometres long and 8 kilometres wide. It's can be seen from space and contains over 1500 types of fish. Jellyfish are well known for their tentacles but in April 2004 there were jellyfish found without tentacles. A shocking find for creatures that can have up to 800 tentacles!
Some cnidarians are used in medical research. Corals and sponges produce chemicals to protect themselves from being infected and overgrown. These chemicals can even provide us with anti-cancer drugs and others may help us learn more about cancer itself.
As you can see, cnidarians are truly amazing. They've been around for more than 650 million years, outdating dinosaurs. Not only are they 95% water, they can be medically benificial, dangerous but interesting, and absolutely stunning.