Planula

n.	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 cœlenterates. It usually has a flattened oval or oblong form, and is entirely covered with cilia.

Noun

1.

planula - the flat ciliated free-swimming larva of hydrozoan coelenterates

hydroid, hydrozoan