http://en.wikipedia.org/wiki/Dolichopteryx_longipes "e brownsnout spookfish, Dolichopteryx longipes, is a species of barreleye in the family Opisthoproctidae. It is the only vertebrate known to employ a mirror, as opposed to a lens, to focus an image in its eyes" D. longipes is unusual in that it utilizes both refractive and reflective optics in sight. The main tubular eye contains a lateral ovoid swelling called a "diverticulum", largely separated from the eye by a septum. The retina lines most of the interior of the eye, and there are two corneal openings, one directed up and the other down, that allow light into the main eye and the diverticulum respectively. The main eye employs a lens to focus its image, as in other fishes. However, inside the diverticulum the light is reflected and focused onto the retina by a curved composite mirror derived from the retinal tapetum, composed of many layers of small reflective plates made of guanine crystals. The split structure of the D. longiceps eye allows the fish to see both up and down at the same time. In addition, the mirror system is superior to a lens in gathering light. It is likely that the main eye serves to detect objects silhouetted against the sunlight, while the diverticulum serves to detect bioluminescent flashes from the sides and below.[1]
