planula

 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.  [1913 Webster]

 The very young, free-swimming larva of the cœlenterates. It usually has a flattened oval or oblong form, and is entirely covered with cilia.  [1913 Webster]