\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. (Zo["o]l.) 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.