The adult eyes are an improvement on invertebrate eyes and were a first step in the development of more advanced vertebrate eyes.
ðɪ ɐdˈʌlt ˈaɪz ɑːɹ ɐn ɪmpɹˈuːvmənt ˌɔn ɪnvˈɜːɾɪbɹˌeɪt ˈaɪz ænd wɜːɹ ɐ fˈɜːst stˈɛp ɪnðə dɪvˈɛləpmənt ʌv mˈoːɹ ɐdvˈænst vˈɜːɾɪbɹˌeɪt ˈaɪz.