Baby Jellies by Peter Willows/BNPS, telegraph.co.uk: Hundreds of moon jellyfish babies have been born at the Weymouth Sealife centre in Dorset. Aquarists say they have never seen so many jelly babies of all shapes, sizes and colours from many different species at one time - but even though they may look cute many of them are highly poisonous as well. #Moon_Jellies
Physalia physalis, a.k.a Portuguese Man'O War. It's not a "simple" jellyfish, but the symbiosis of more specialized foms of life. In the best case, its urticant, toxic touch burns like a flame [TLDR version: pretty, but don't touch!]
Anyone unfamiliar with the biology of the venomous Portuguese man-of-war would likely mistake it for a jellyfish. Not only is it not a jellyfish, it's not even an "it," but a "they." The Portuguese man-of-war is a siphonophore, an animal made up of a colony of organisms working together.