The Collaborative International Dictionary of English v.0.48:

Pyrosome \Pyr"o*some\, n. [Pyro- + -some body.] (Zool.)
   Any compound ascidian of the genus Pyrosoma. The pyrosomes
   form large hollow cylinders, sometimes two or three feet
   long, which swim at the surface of the sea and are very
   phosphorescent.
   [1913 Webster]