Siphonophores are hydrozoan colonies in which a single fertilised egg produces a stem that buds zooids, each specialised for one task and none capable of living alone. Swimming bells drive the colony, feeding polyps digest, tentacle-bearing zooids fish, and reproductive zooids make gametes.
In the physonect and calycophoran groups the stem can become extraordinarily long. Praya dubia has been reported at around forty to fifty metres, and a specimen of the genus Apolemia filmed off Western Australia in 2020 was estimated by the observing team to have an outer ring roughly 47 metres across, with the total colony longer still. These are among the longest animals recorded by any measure.
The qualifier matters. Length here describes a filamentous colony a few centimetres thick, not a body with a blue whale's mass. Siphonophores also break apart under almost any handling, so most historical estimates come from submersible video rather than captured specimens. The frequent shorthand that a siphonophore is the longest animal is defensible only once the colonial nature and the measurement conditions are stated.