Velella \Ve*lel"la\, n. [NL., dim. from L. velum a veil, a sail.] (Zo["o]l.) Any species of oceanic Siphonophora belonging to the genus {Velella}. Note: These creatures are brilliantly colored and float at the surface of the sea. They have an oblong, disklike body, supported by a thin chitinous plate, from which rises a thin diagonal crest which acts as a sail. The feeding and reproductive zooids hang down from the under side of the disk.