Is there such a thing as evolution of the eye? Creationists and evolutionists have debated over this topic for years. But let's not entangle ourselves in this debate. Instead let's explore the gradual change in visual perception from simple to complex organisms.
The main categories and representatives of each of them are as follows:
Euglena is a unicellular organism. It lives in a water medium and moves by beating its flagellum.
The light sensitive eyespot is called stigma.
1. located at the anterior end of the organism
2. contains a red pigment
Use: Light directs the movement of Euglena, which orients itself to expose photo-sensitive cells to light (a phototactic response).
1.cup-shaped, heavily-pigmented cells as retina
2.sense cells in contact with retina cells
Use: it can differentiate the direction and the intensity of light because cup-shaped pigment cells of the eyes shield the light sensitive cells in all directions with only one opening for light entry.
Nautilus is an animal with a shell (cephalopoda) and lives in the ocean bottom of low light intensity.
Features: with retina, no cornea, no lensUse: The pinhole eye brings about poor resolution and images formed are dim. It is suitable for the living habitat of Nautilus.
Compound eyes can be found in all insects whilst simple eyes are found mostly in larva and also acts as an accessory in some flying insects.
|Compound eye||Simple eye|
|made up of thousands of units called ommatidia
|alias ocellus which is similar to a simple ommatidia
cuticular lens with light sensitive cells underneath joined to the brain by a single nerve
|good for noticing movements||good for reacting to fluctuations in light intensity|
Human eyes have often been compared to cameras. They are alike in terms of structure, but they have one fundamental difference in functioning mechanism.
|1.||opening for light to enter||aperture||pupil|
|2.||control the amount of light entering camera/eye||diaphragm control size of aperture||iris muscles control size of pupil|
|3.||refract light||glass biconvex lens||mainly cornea ;
lens, aqueous & vitreous humor
|4.||object of light action to form image||photosensitive chemicals on film||photoreceptors(rods & cones) in retina|
|5.||absorb excessive light to prevent multiple images formation||dark internal surface||pigmented, dark choroid|
|1.||focusing mechanism||change distance between lens & film||change focal length of lens using ciliary muscles|