Gastraea Theory

a theory formulated by E. Haeckel (based to a considerable degree on the comparative embryological studies of A. O. Kovalevskii), according to which all multicellular animals originated from a single common ancestor. Haeckel called this hypothetical organism the gastraea. According to his theory, the gastraea had an oval, sacciform body with a two-layered wall and an oral aperture; he pictured the outer layer as the skin and the inner as the gut wall. The gastraea arose in the process of evolution by the invagination of a single-layered bubble-shaped animal, which Haeckel called the blastaea. He believed that the two-layered embryo of the gastrula stage repeats the structure of this common ancestor, following what he called the biogenetic law. He thought that of all contemporary animals the coelenterates are closest to the gastraea. The gastraea theory had few adherents since there is no basis for believing that invagination was a primitive means of gastrulation.