All vertebrates have eyes that are similar in basic construction plan. This eye must have evolved very early in chordate evolution and then got modified subsequently to increase its efficiency in higher vertebrates.
Vertebrate eye is a hollow ball made of three layers, outermost of which is called sclera that is made of bone, cartilage or fibrous tissue for protection of this delicate and important organ. The middle layer is called choroid which is dark and vascularised layer and the innermost layer is composed of sensory cells, called retina. Sclera is transparent in front of the eye ball to make cornea that allows the light rays to enter the eye ball.
Choroid layer in front makes the iris diaphragm and the ciliary bodies that carry muscles and glands and also suspend a crystalline lens by suspensory ligaments. The space in front of the lens is filled with a watery fluid called aqueous humour and the chamber posterior to the lens is filled with a gelatinous substance called vitreous humour that makes 80% of the eye volume. Cornea is covered in front of the eye ball with modified skin called conjunctiva which is vascularised layer for protection as well as nourishing cornea. There are many glands associated with the eye, namely, lacrimalglands, Meibomian glands,glands of Moll or Zeiss and Harderian glands. Eye lids protect and wipe the eye ball clean and in some animals an additional nictitating membrane is present for protection.
Retina is made of sensory rod and cone cells, which are more concentrated on the area centralis and the fovea centralis. Rod cells give coarse and black and white vision while cone cells provide colours and sharp vision. As the light falls on these sensory cells a chemical reaction breaks down the pigments rhodopsin (in rods) or iodopsin (in cones) into scotopsin or photopsin respectively and retinene, which generates a nerve impulse that is carried to the brain via the optic nerve.
Protochordates do not have true eyes; Amphioxus has pigmented eye spots on the ventro-lateral sides of the spinal cord for detecting light and shades.
CYCLOSTOME EYE
Hagfishes have rudimentary eyes that are hardly one mm in diameter but Petromyzon has well developed eyes. The eye ball is flat and eye lids are absent. Lens is permanently spherical and there are no ciliary bodies. Sclera and choroid are fused and pupil has fixed diameter.
FISH EYE
Eyelids are immovable and permanently open and nictitating membrane covers the eye in front. In elasmobranchs, cornea is flat in front, lens permanently spherical which is moved forward and backward by protectorlentis muscle. There is no fovea in area centralis but there is a layer of shiny guanine crystals called tapetum behind retina for night vision. Sclera is cartilaginous in elasmobranchs.
In bony fishes also the eye ball is elliptical and lens permanently spherical that almost touches the cornea in front. Lens is moved forward and backward by a muscle called campanulaHalleri that is attached to the processusfalciformis at the back of the eye ball. There is no fovea on retina and the layer of reflecting cells is called argentea.
AMPHIBIAN EYE
Limbless amphibians have rudimentary eyes owing to their burrowing habits. Urodeles also have small eyes which are essentially fish like in structure. However, anurans have evolved an eye that is suitable for vision in terrestrial conditions.
Frog eye is oval in shape with three distinct layers, fibrous sclera, vascular choroid and sensory retina. Eye lids are movable and lower eye lid is larger and also carries a nictitating membrane. Lachrymalglands are also located on the lower side of the eye ball and a harderiangland is present on the back of the eye ball. Lens is flat on the front side but convex on the posterior side. Ciliary body is well developed but lens is moved forward and backward for focusing by a protractorlentis muscle. The whole eye ball can be pulled back into the eye orbit by a retractorbulbi muscle and can be brought forward by levatorbulbi muscle. Frog possesses limited colour vision.
REPTILIAN EYE
Reptilian eye is made on amphibian plan and is adapted for terrestrial environs. Sclera is cartilaginous or bony and retina possesses a foveacentralis. Lizards also have a conuspapillaris protruding on the lower side of the fovea. It probably has nourishing function. Lens is soft and flexible and can alter focal length by squeezing with the help of ciliary muscles. Lachrymal glands are well developed except in Sphenodon and snakes. Lower eye lid is still larger and nictitating membrane is present. In snakes eyelids are fused and become transparent to form a spectacle in front of the eye. Crocodiles have a pecten as in birds. Colour vision seems to be present in some lizards and turtles.
BIRD EYE
Bird eye is best developed among all vertebrates which is a necessity for a flying animal. Bird eye ball is large and depressed in the area of powerful ciliary body. Sclerotic coat is tough with cartilages and bones. Retina is highly sensitive having abundance of cones and a very deep foveacentralis. A pecten is present in birds which increases contrast and reduces glare in bright sunshine. Lens is highly flexible and can be squeezed by two powerful muscles, namely, muscle of Crampton and muscle of Bruke. Iris is supplied with striated muscles. Nictitating membrane is used to cover the front of eyeball when the bird is in flight. Except for the nocturnal birds, all birds possess colour vision.
MAMMALIAN EYE
In mammals sclera is made of fibrous tissue and continues in front as cornea. Retina has a fovea but there is no pecten. Iris can change the diameter of pupil by radial and circular muscles. Lens is soft and capable of changing focal length by altering its thickness with the help of circular and meridional muscles present in the ciliary body. Upper eyelid is large in mammals and lachrymal gland is located on the upper side of the eyeball. Nictitating membrane is present in some mammals such as horse, anteaters, panda and caribou. A modification of iris called umbraculum is found in cats, camels etc. which cuts excessive glare in daytime.