Cephalopod eye

rdf:langString Cephalopod eye

rdf:langString In vertebrate eyes, the nerve fibers route before the retina, blocking some light and creating a blind spot where the fibers pass through the retina. In cephalopod eyes, the nerve fibers route behind the retina, and do not block light or disrupt the retina. 1 is the retina and 2 the nerve fibers. 3 is the optic nerve. 4 is the vertebrate blind spot.

rdf:langString right

rdf:langString 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 the cornea lies over the top of the eye as opposed to being a structural part of the eye. 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. Cephalopods' eyes develop in such a way that they have retinal axons that pass over the back of the retina, so the optic nerve does not have to pass through the photoreceptor layer to exit the eye and do not have the natural, central, physiological blind spot of vertebrates. The crystalins used in the lens appear to have developed independently from vertebrate crystalins, suggesting a homoplasious origin of the lens. Most cephalopods possess complex extraocular muscle systems that allow for very fine control over the gross positioning of the eyes. Octopuses possess an autonomic response that maintains the orientation of their pupils such that they are always horizontal.