Velella velella floats on an oval chitinous disc supported by concentric gas-filled chambers, with a stiff vertical sail rising from it. The sail does not run along the long axis of the float; it is set diagonally, at roughly forty-five degrees, and that angle determines how the animal moves relative to the wind.
Both chiralities occur. Right-handed and left-handed individuals are mirror images, and each is deflected to a different side of the downwind direction, so a persistent wind sorts a mixed population into two diverging streams. This has been offered as an explanation for why strandings on a given coast are often dominated by one form, and it spreads the risk that a single weather pattern beaches the entire population.
Velella is a hydrozoan colony related to the Portuguese man o' war, with feeding, reproductive and defensive zooids under the float, and it releases tiny medusae. Its sting is negligible to humans, unlike its larger relative. The blue pigment fades quickly on the beach, so a stranding line often looks like scattered clear plastic within a day or two.