Lamarck, 1801 
Physalia is a genus of the order Siphonophora, colonies of four specialized polyps and medusoids that drift on the surface of the Atlantic, Indian and Pacific oceans. A gas-filled bladder resembling a blue bottle provides buoyancy, and long tentacles of venomous cnidocytes provide a means of capturing prey. A sail on the float, which may be left or right-handed, propels Physalia about the sea, often in groups. Individuals sometimes become stranded on beaches, where their toxic nematocysts can remain potent for weeks or months in moist conditions.
The species Physalia utriculus is given the common name Pacific man o' war to distinguish it from the more widely distributed and larger Physalia physalis, the Portuguese man o' war. The species are told apart by the size of the float (six inches compared to twelve) and by having a single versus several long fishing tentacles. No fatalities from envenomation are recorded for P. utriculus, in contrast to the larger species.
The family Physaliidae is monotypic, consisting of only one genus, Physalia. The genus contains only two species.