The Collaborative International Dictionary of English v.0.48:

Physalia \Phy*sa"li*a\, prop. n. [NL., fr. Gr. fysalli`s a
   bladder, fr. fy^sa a bellows.] (Zool.)
   A genus of large oceanic Siphonophora which includes the
   Portuguese man-of-war.
   [1913 Webster]

   Note: It has a large air sac, or float, with a sail-like
         crest on its upper side. Numerous zooids of different
         kinds are attached to the under side of the float. Some
         of the zooids have very long tentacles; some have a
         mouth and digest food; others produce gonophores. The
         American species (Physalia arethusa) is brilliantly
         colored, the float being pink or purple, and bright
         blue; the zooids blue. It is noted for its virulent
         stinging powers, as well as for its beautiful colors,
         graceful motions, and its ability to sail to windward.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Physalia
    n 1: Portuguese man-of-war [syn: Physalia, genus Physalia]