The Collaborative International Dictionary of English v.0.48:

Velella \Ve*lel"la\, n. [NL., dim. from L. velum a veil, a
   sail.] (Zool.)
   Any species of oceanic Siphonophora belonging to the genus
   Velella.
   [1913 Webster]

   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.
         [1913 Webster]