Siphonophores include large hydrozoans such as the Portuguese Man-Of-War (Physalia physalis), or Praya dubia, which can be as long as several dozen meters. These large animals are actually colonies of many, smaller individuals (polyps). While each polyp is a separate animal, they play such specialized roles that they function almost like organs within a single animal. For example, some polyps in the Portuguese Man-Of-War (gastrozooids) are specialized for eating. Prey goes into their mouths, and the nutrients digested from it are circulated to other polyps in the colony. These feeding polyps rely on the tentacle polyps (dactylozooids) to actually catch the food. An oversized, gas-filled polyp (pneumatophore) keeps the colony afloat and drifting in the ocean currents. Reproductive polyps (gonozooids) make eggs or sperm. Some siphonophore polyps may not even be able to survive independently, which calls into question whether they can really be considered individual animals.

Like their close relatives, hydroids have a life cycle that includes two body types. The polyp type is stationary, anchoring on the ocean bottom or other substrate. The medusa type swims around and looks like a jellyfish. In hydroids, the polyp type tends to be more common, although many species have both types. Polyps typically form colonies containing up to thousands of small individuals. A colony usually has either all male or all female polyps. One polyp may start a colony, then enlarge it by budding off new polyps (asexual reproduction). Some mature polyps have the special ability to make mini medusae that detach and swim away. When they are grown up, female medusae release eggs into the sea, while male medusae release sperm (sexual reproduction). A fertilized egg becomes a larva that either develops into another medusa or into a polyp. A new polyp anchors, which restarts the cycle.