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.