Portuguese man-of-war

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A colonial siphonophore (Physalia physalis) composed of numerous specialized zooids, having a bluish bladderlike crested float from which hang numerous long stinging tentacles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A floating colony of hydrozoans (Physalia physalis) attached to a float; it superficially resembles a jellyfish.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun large siphonophore having a bladderlike float and stinging tentacles