The Collaborative International Dictionary of English v.0.48:

Gastrula \Gas"tru*la\, n.; pl. Gastrul[ae][NL., dim. fr. Gr. ?
   the stomach.] (Biol.)
   An embryonic form having its origin in the invagination or
   pushing in of the wall of the planula or blastula (the
   blastosphere) on one side, thus giving rise to a
   double-walled sac, with one opening or mouth (the blastopore)
   which leads into the cavity (the archenteron) lined by the
   inner wall (the hypoblast). See Illust. under Invagination.
   In a more general sense, an ideal stage in embryonic
   development. See Gastr[ae]a. -- a. Of or pertaining to a
   gastrula.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Planula \Plan"u*la\, n.; pl. Planul[ae]. [L., a little plane.]
   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]

The Collaborative International Dictionary of English v.0.48:

Stomapoda \Sto*map"o*da\, n. pl. [NL. See Stoma, and -poda.]
   (Zool.)
   An order of Crustacea including the squillas. The maxillipeds
   are leglike in form, and the large claws are comblike. They
   have a large and elongated abdomen, which contains a part of
   the stomach and heart; the abdominal appendages are large,
   and bear the gills. Called also Gastrula, Stomatopoda,
   and Squilloidea.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gastrula
    n 1: double-walled stage of the embryo resulting from
         invagination of the blastula; the outer layer of cells is
         the ectoderm and the inner layer differentiates into the
         mesoderm and endoderm