planula

Plan"u*la, n.; pl. Planulæ. Etym: [L., a little plane.] 1. (Biol.) Defn: 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. (Zoöl.) Defn: The very young, free-swimming larva of the coelenterates. It usually has a flattened oval or oblong form, and is entirely covered with cilia.

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