The Collaborative International Dictionary of English v.0.48:

Pennatulacea \Pen*nat`u*la"ce*a\, n. pl. [NL. See Pennatula.]
   (Zool.)
   A division of alcyonoid corals, including the seapens and
   related kinds. They are able to move about by means of the
   hollow muscular peduncle, which also serves to support them
   upright in the mud. See Pennatula, and Illust. under
   Alcyonaria.
   [1913 Webster]