Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

planula
     n : the flat ciliated free-swimming larva of hydrozoan
         coelenterates
     [also: {planulae} (pl)]

planulae
     See {planula}