They are usually deep blue in colour, but their most obvious feature is a small stiff sail that catches the wind and propels them over the surface of the sea.

They catch their prey, generally plankton, by means of tentacles that hang down in the water and bear cnidocysts (also called nematocysts).

[8] Having no means of locomotion other than its sail, V. velella is at the mercy of prevailing winds for moving around the seas, and are thereby also subject to mass-strandings on beaches throughout the world.

[11] The Porpitidae is a family of the Hydrozoa erected for two genera of hydroids that live floating free at the surface of the open ocean: Velella and Porpita.

The systematic position of these peculiar genera has long been a topic of discussion among taxonomists who work with pelagic Cnidaria.

The three genera[clarification needed] were put in with athecate hydroids in the mid-to-late 19th century by some, whereas other authors at the time included them in the Siphonophorae.

[11] Most authors in the past 40 years have accepted interpretation of these animals as unusual floating colonial athecate hydroids, which produce medusae clearly belonging in the Anthomedusae.

Although the exact position of the family Porpitidae within the Athecatae/Anthomedusae is not yet clear, the order Chondrophora is no longer used by hydrozoan systematists.