The Jellyfish, whilst not actually a fish belongs to the phylum Cnideria (pronounced nigh-deer-e-a) and includes several thousand different species.
Many jellyfish are capable of 'glowing' or bio-luminescence. This is due to the presence of fluorescent proteins.
Jellyfish lack fins, and move via the undulation of many limb-like tentacles. These tentacles often carry barbs, which are toxic to most animals and humans.
Pictured: A jellyfish nemopilema.