planul[ae]

Webster's Dictionary (GCIDE)

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. [1913 Webster]

2.(Zool.) 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. [1913 Webster]