planula

Etymology
, a little plane.

Noun

 * 1)  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.
 * 2)  The larva of a hydrozoan, which is free-swimming and covered in cilia.