Heterogeneous eyes have evolved at least nine Times.

http://en.wikipedia.org/wiki/Eye

Heterogeneous eyes have evolved at least nine times: four or more times in gastropods, once in the copepods, once in the annelids, once in the cephalopods,and once in the chitons, which have aragonite lenses.No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".