Eyes are categorized into two types, "simple eyes" and "compound eyes". In this case, "simple eyes" do not mean that they have reduced complexity or acuity, but rather that they have one concave photoreceptive surface. This is in contrast to "compound eyes" which are comprised of a number of individual lenses on a convex surface. In other words, eyes fall into two groups based on their photoreceptor's cellular construction either being cilliated (as in the vertebrates) or rhabdomeric.