The Collaborative International Dictionary of English v.0.48:

Gastraea \Gas*tr[ae]"a\, n. [NL., from Gr. ?, ?, the stomach.]
   (Biol.)
   A primeval larval form; a double-walled sac from which,
   according to the hypothesis of Haeckel, man and all other
   animals, that in the first stages of their individual
   evolution pass through a two-layered structural stage, or
   gastrula form, must have descended. This idea constitutes the
   Gastr[ae]a theory of Haeckel. See Gastrula.
   [1913 Webster]