Over time, three different types of eyes have evolved: a single-lens eye, a mirror eye, and a compound eye (Gehring 2001). Both cephalopods and vertebrates have a visual system based on a single-lens eye type which explains their immense similarity (Lane 1960). In a single-lens eye type, light enters in through the pupil and is focused on the photoreceptor-containing retina by the lens. Cephalopods however, have a pupil that is rectangular in shape and changes between a square or narrow slit, as opposed to the vertebrate pupil that is circular and changes only in diameter. The amount of light allowed to enter into the eye at any time, is controlled by the iris which regulates the size of the pupil. The cephalopod lens though is much more spherical than the vertebrate lens, with a focal length 2.5 times the lens radius, and focuses on objects in different ways as well (Muntz 1991). Unlike vertebrates that change the shape of their lens to focus on near or distant objects, cephalopods use their ciliary muscles to draw the lens in, in order to see distant objects and leaves the lens relaxed to focus on objects close to them (Young 1971, Wells 1962). The exterior of both eye types are surrounded and protected by a sclera, which is a layer of cartilage made up of cells and two sheets of matrix (Williams). Depending on the species of Cephalopods, the eye itself is controlled and suspended into the eye socket by nine to fourteen different muscles (Budelmann and Young 1993, Williams).

Image 4: The Eye and Retinal Strucutre of a Vertebrate and Cephalopod (Image used by permission of May'ayan Semo- Imperial College London; from Semo, Ma'ayan. 1998. Evolution of the Eye: Morphology and Developmental Patterns. http://www.maayan.uk.com/evoeyes1.html. October 2, 2003. Image taken from: Halder, G., P. Callaerts, and W.J. Gehring. New Perspectives on Eye Evolution. Current Opinions In Genetics & Development. 1995; 5:602-60.