Velella velella is not an independent body, and a colony of hydroids.
Inside the colonies formed flat chamber filled with air from chitin, which is covered with a mantle. It was she who maintains the entire colony on the water surface. A characteristic feature of the type is a thin semi-circular blade, located diagonally from the top of the air chamber. Rises above the base of the sail, which is located asymmetrically and is curved in the shape of the letter S. With this blade animal has been called (from the Latin. «Velum» - sail). This allows the animal to move outgrowth on the surface of the water is not in a straight line, and at a certain angle to the wind, periodically turning 180 degrees. Colony size can reach 10 cm (typically about 7 cm.). Threadlike tentacles, relatively short, hanging from the air chamber. Their number varies from 8 or more.
Polyps form colonies have different shapes and specialization. There is a pronounced central large polyp, which is gastrozooidom (specializes in catching and digesting food). It is surrounded by gastro-gonozooids (polyps, specializing in nutrition and reproduction) and daktilozooidami (polyps, specializing in protective function).