GastraeaGas*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.