The Collaborative International Dictionary of English v.0.48:

Pennatula \Pen*nat"u*la\, n.; pl. L. Pennatul[ae], E.
   Pennatulas. [NL., fr. L. penna a feather.] (Zool.)
   Any one of numerous species of Pennatula, Pteroides, and
   allied genera of Alcyonaria, having a featherlike form; a
   sea-pen. The zooids are situated along one edge of the side
   branches.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Pennatula
    n 1: type genus of the family Pennatulidae: sea pens [syn:
         Pennatula, genus Pennatula]