An Agalma okeni taken out of the water so that it is possible to see all the gelatinous parts.

Siphonophores belong to the Cnidaria, a group of animals that includes the corals, hydroids, and true jellyfish. There are about 175 described species. Some siphonophores are the longest animals in the world, and specimens as long as 40 meters have been found. The majority of siphonophores are long and thin, consisting mostly of a clear gelatinous material. Some deep water species have dark orange or red digestive systems that can be seen inside their transparent tissues. Siphonophores are exceedingly fragile and break into many pieces under even the slightest forces. Many siphonophores are bioluminescent, glowing green or blue when disturbed. All siphonophores are predators, and use their many tentacles to capture crustaceans and small fish.

While one species of siphonophore lives at the surface of the ocean (the familiar Portuguese Man O' War, Physalia physalis), and members of another group (the Rhodaliids) tethered themselves to the bottom with their tentacles, the vast majority of siphonophores are active swimmers and live in the water column of the open ocean. A few hardy species are sometimes found near the shore, but these are the exception.