According to Official Guinness Records,
Although they superficially resemble true jellyfish and are distantly related to them, siphonophores are very different, special creatures. Remarkably, each individual siphonophore is actually a colony or super-organism, i.e. each part of its body is in reality a distinct highly specialized organism or zooid in its own right, adapted anatomically to fulfil a certain specific function (e.g. floating, stinging, feeding, reproducing). In other words, each of its tentacles is a separate organism, as is its float, each of its reproductive organs, each of its bells, etc. One species of siphonophore, Praya dubia, is considered to be the longest animal in the world measuring between 30–50 m (100–160 ft) in length. It has large paired transparent swimming bells at the head and trailing behind is a long stem made up of units called cormidia which it pulls through the water. Its thin tentacles can deliver a powerful sting and it also produces a blue bioluminescent glow. Praya are among the most commonly sighted siphonophores in the waters of central California and are identified by its bright yellowy stem colour.
By way of comparison the blue whale is between 25-27 m long.
For a complete list of 2002 records, please visit 2002 Guinness Records in United States.