The Collaborative International Dictionary of English v.0.48:

Porpita \Por"pi*ta\, n. [NL., from Gr. ? brooch.] (Zool.)
   A genus of bright-colored Siphonophora found floating in the
   warmer parts of the ocean. The individuals are round and
   disk-shaped, with a large zooid in the center of the under
   side, surrounded by smaller nutritive and reproductive
   zooids, and by slender dactylozooids near the margin. The
   disk contains a central float, or pneumatocyst.
   [1913 Webster]