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Bilaterians likely evolved from an ancestor that was radially symmetrical. There have been suggestions that the blastopore started out as the digestive surface on a radial organism that became elongated (and thus bilaterally symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. This matches with the "flaps-folding-over" model of gut formation, but an alternative view is that the original blastopore migrated forwards to one end of the ancestral organism before deepening to become a blind gut. This is

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