The Collaborative International Dictionary of English v.0.48:

Planula \Plan"u*la\, n.; pl. Planul[ae]. [L., a little plane.]
   1. (Biol.) In embryonic development, a vesicle filled with
      fluid, formed from the morula by the divergence of its
      cells in such a manner as to give rise to a central space,
      around which the cells arrange themselves as an envelope;
      an embryonic form intermediate between the morula and
      gastrula. Sometimes used as synonymous with gastrula.
      [1913 Webster]

   2. (Zool.) The very young, free-swimming larva of the
      c[oe]lenterates. It usually has a flattened oval or oblong
      form, and is entirely covered with cilia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

planula
    n 1: the flat ciliated free-swimming larva of hydrozoan
         coelenterates