Cephalopods, as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a
camera-type eye which consists of an iris, a circular lens, vitreous cavity (eye gel), pigment cells, and photoreceptor cells that translate light from the light-sensitive
retina into nerve signals which travel along the optic nerve to the brain. For the past 140 years, the camera-type cephalopod eye has been compared with the vertebrate eye as an example of
convergent evolution, where both types of organisms have independently evolved the camera-eye trait and both share similar functionality. Contention exists on whether this is truly convergent evolution or
parallel evolution. Unlike the
vertebrate camera eye, the cephalopods' form as
invaginations of the body surface (rather than outgrowths of the brain), and consequently they lack a
cornea. Unlike the vertebrate eye, a cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The
eye is approximately spherical, as is the
lens, which is fully internal.