▁from ▁an ▁ancest or ▁which ▁was ▁radial ly ▁symmet rical . ▁There ▁have ▁been ▁suggestions ▁that ▁the ▁bl ast op ore ▁started ▁out ▁as ▁the ▁dig est ive ▁surface ▁on ▁a ▁radial ▁organ ism ▁which ▁became ▁el ong ated ▁( and ▁thus ▁bil aterial ly ▁symmet rical ) ▁before ▁its ▁sides ▁closed ▁over ▁to ▁leave ▁a ▁mouth ▁at ▁the ▁front ▁and ▁an ▁an us ▁at ▁the ▁rear . ▁This ▁matches ▁with ▁the ▁" f laps - fold ing - over " ▁model ▁of ▁gut ▁formation , ▁but ▁an ▁alternative ▁view ▁is ▁that ▁the ▁original ▁bl ast op ore ▁migr ated ▁for wards ▁to ▁one ▁end ▁of ▁the ▁ancest ral ▁organ ism ▁before ▁deep ening ▁to ▁become ▁a ▁blind ▁gut . ▁This ▁is ▁consistent ▁with ▁living ▁X en aco el om or pha , ▁which ▁are ▁the ▁sister ▁tax on ▁to ▁proto st om es ▁and ▁de uter ost om es . ▁The ▁story ▁is ▁a ▁little ▁more ▁complex , ▁because ▁the ▁bl ast op ore ▁itself ▁does ▁not ▁directly ▁give ▁rise ▁to ▁the ▁mouth ▁of ▁these ▁w orm s . ▁This ▁suggests ▁that ▁the ▁last ▁common ▁ancest or ▁of ▁bil ater ians ▁had ▁a ▁similar ▁gut ▁configuration , ▁and ▁that ▁the ▁an us ▁evol ved ▁after ▁the ▁mouth . ▁ ▁Ex actly ▁how ▁a ▁through ▁gut ▁formed ▁from ▁this ▁blind ▁gut ▁is ▁somewhat ▁harder ▁to ▁tell . ▁The ▁gen etic ▁mechan isms ▁responsible ▁for ▁an us ▁formation ▁are ▁quite ▁variable , ▁which ▁might ▁suggest ▁that ▁the ▁an us ▁evol ved ▁several ▁times ▁in