Gastraa or Gastraea: How To spell?

The word Gastraa is misspelled against Gastraea, a noun meaning "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 Gastraea theory of Haeckel. See Gastrula."