▁thought ▁that ▁the ▁bl ast op ore ▁of ▁the ▁proto st om es ▁formed ▁the ▁mouth , ▁and ▁the ▁an us ▁was ▁formed ▁second ▁when ▁the ▁gut ▁tun ne led ▁through ▁the ▁emb ry o . ▁More ▁recent ▁research ▁has ▁shown ▁that ▁our ▁understanding ▁of ▁proto st ome ▁mouth ▁formation ▁is ▁somewhat ▁less ▁secure ▁than ▁we ▁had ▁thought . ▁A co el om or pha , ▁which ▁form ▁a ▁sister ▁group ▁to ▁the ▁rest ▁of ▁the ▁bil ater ian ▁animals , ▁have ▁a ▁single ▁mouth ▁that ▁leads ▁into ▁a ▁blind ▁gut ▁( with ▁no ▁an us ). ▁The ▁gen es ▁employed ▁in ▁the ▁emb ry onic ▁construction ▁of ▁the ▁flat w orm ▁mouth ▁are ▁the ▁same ▁as ▁those ▁expressed ▁for ▁the ▁proto st ome ▁and ▁de uter ost ome ▁mouth , ▁which ▁suggests ▁that ▁the ▁structures ▁are ▁equivalent ▁hom olog ous , ▁and ▁that ▁the ▁older ▁ideas ▁about ▁proto st ome ▁mouth ▁formation ▁were ▁correct . ▁An ▁alternative ▁way ▁to ▁develop ▁two ▁open ings ▁from ▁the ▁bl ast op ore ▁during ▁g astr ulation , ▁called ▁am ph ist omy , ▁appears ▁to ▁exist ▁in ▁some ▁animals , ▁such ▁as ▁n emat odes . ▁ ▁In ▁humans ▁the ▁per for ation ▁of ▁the ▁an us ▁and ▁mouth ▁happen ▁at ▁ 8 ▁weeks ▁and ▁ 4 ▁weeks ▁respectively . ▁When ▁both ▁the ▁an us ▁and ▁mouth ▁are ▁per for ated , ▁the ▁organ ism ▁becomes ▁top olog ically ▁a ▁tor us . ▁ ▁E volution ary ▁origin ▁ ▁Bil ater ians ▁likely ▁evol ved