1. the flat ciliated free-swimming larva of hydrozoan coelenterates;
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.
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.
WordNet (r) 3.0 (2006):
n 1: the flat ciliated free-swimming larva of hydrozoan