Like other cnidarians, Velella velella is a gelatinous creature. However,unlike some kinds of jellyfish, the "fin" is more rigid, allowing it to catch the wind and sail on the surface of the water. Velella velella is one of the organisms that lives at the air-sea interface as part of pleuston. Due to it's mode of transportation, its common name is By-the-wind-sailor.
Occasionally, huge numbers of these organisms wash up on the beach along the southern California coastline.
Photo used with permission from CalPhotos, Digital Image Project, UC Berkeley, photographer, Sherry Ballard.