Definition of "gastraea" [‖Gas•træ•a]

• 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 Gastræa theory of Haeckel. See gastrula. (noun)

Gnu Collaboartive International Dictionary of English: licensed under The Code Project Open License (CPOL)

• "We shall make use of it later on for our monophyletic theory of descent -- the hypothesis of a common descent of man and all the metazoa from the gastraea."
• "The gastraea theory shows us how to do this, by representing the embryology of the lowest vertebrate, the skull-less amphioxus, as the original form, and deducing from it, through a series of gradual modifications, the gastrulation and coelomation of the craniota."
• "Hence in the light of the gastraea theory we must regard the features of the amphioxus as the only and real primitive structure among all the vertebrates, departing very little from the palingenetic embryonic form."