A primeval larval form; a double-walled sac from which, based on the hypothesis of Haeckel, guy and all various other pets, that in the first stages of these individual advancement pass through a two-layered architectural phase, or gastrula kind, must-have descended. This notion constitutes the Gastræa concept of Haeckel. See gastrula.
In biology, a hypothetical pet type believed by Haeckel because the ancestor of all metazoic animals — that's, of those which go through or achieve on morphological kind of a gastrula. See gastrula.