Jellyfish, sometimes known as true jellyfish, are a class of cnidarians found in every ocean of the world from the surface to the seabed. Most are free-swimming, but some are sessile and grow fixed to the seabed. Because jellyfish can't swim against a current, they are classed as planktonic animals - even when they're the size of the huge lion's mane jellyfish, which can grow to over 35m long. Box jellyfish and the Portuguese man o'war are not true jellyfish, although they are related to them.
Scientific name: Scyphozoa
Discover the other animals and plants that lived during the following geological time periods.