[2] Controversy surrounds who the sister groups of Hydroidolina are, but research has shown that three orders remain consistent as direct relatives: Siphonophorae, Anthoathecata, and Leptothecata.

[4] Hydroidolina are small predatory animals, ranging in 8-30 millimeters in size,[5] exhibiting radial symmetry and are diploblastic (developed from two embryonic layers: ectoderm and endoderm).

[citation needed] Benthic polyps can be found on a variety of hard substrates, including both natural and artificial surfaces.

[11][12] For example, the bivalve mantle cavity of a mollusk provides a sheltered environment, transporting food due to the current.

[17] The exoskeleton varies in chemical composition, structural rigidity, thickness, and coverage within the different regions of the colony and protects the coenosarc of the polypoid stage.

The obsolete name Hydroida was used for a paraphyletic grouping that is now considered synonymous with Hydroidolina but did not include the colonial jellies of the order Siphonophorae.

[22][23] These sessile invertebrates could prove to be useful as a measure of environmental changes within their own colonies as well as for changes within near marine environments pertaining to temporal and spatial changes to species distribution and composition, temperature, and food.