sˈaɪəntɪsts dˌɪsɐɡɹˈiː ɐbˌaʊt ðˈɪs: dʒˈɪɹɪbɪt ænd kˈɑːliːɡz kəŋklˈuːdᵻd, ɪn tˈuː θˈaʊzənd sˈɪks, ðə ɹˌɛpətˈɪʃən ʌv ɡˈɪlz ænd ʌvðə fˈʊts ɹᵻtɹˈæktɚ mˈʌsəlz wɜː lˈeɪɾɚ dɪvˈɛləpmənts, wˌaɪl ɪn tˈuː θˈaʊzənd sˈɛvən, sˈɪɡwɔːɹt kəŋklˈuːdᵻd ðɪ ænsˈɛstɹəl mˈɑːlʌsk wʌz mˌɛɾəmˈɛɹɪk, ænd ɪt hæd ɐ fˈʊt jˈuːzd fɔːɹ kɹˈiːpɪŋ ænd ˈeɪ "ʃˈɛl" ðæt wʌz mˈɪnɚɹəlˌaɪzd.
Scientists disagree about this: Giribet and colleagues concluded, in 2006, the repetition of gills and of the foot's retractor muscles were later developments, while in 2007, Sigwart concluded the ancestral mollusc was metameric, and it had a foot used for creeping and a "shell" that was mineralized.