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:

• Can only detect absence and presence of light
  • flat light sensitive patches (simple photoreceptors): Euglena
  • cupped light sensitive spots: Planaria
• With definite images
  • pinhole with no lens: Nautilus
  • simple and compound eye : arthropoda
  • camera eyes (with lens): vertebrate e.g. humans ; cephalopoda e.g.octopus, squid

LIGHT SENSITIVE EYE PATCHES ----- e.g. Euglena

Euglena is a unicellular organism. It lives in a water medium and moves by beating its flagellum.

The light sensitive eyespot is called stigma.

Features:
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).

SIMPLE EYE-CUP WITHOUT LENS ----- e.g. Planaria

Features:
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.

PINHOLE EYES ----- e.g. Nautilus

Nautilus is an animal with a shell (cephalopoda) and lives in the ocean bottom of low light intensity.

Features: with retina, no cornea, no lens

COMPOUND EYE VS. SIMPLE EYE ----- e.g. insects

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.

Table: Comparison between compound eye and simple eye

Compound eye	Simple eye
made up of thousands of units called ommatidia (singular: ommatidium)	alias ocellus which is similar to a simple ommatidia cuticular lens with light sensitive cells underneath joined to the brain by a single nerve
3D image formed, but not sharp with colour mosaic image (the resultant of images from ommatidium pointing in a slightly different direction)	no image formed not good in colour discrimination sensitive to different degrees of light intensity
good for noticing movements	good for reacting to fluctuations in light intensity

HUMAN EYES vs. CAMERA

Human eyes have often been compared to cameras. They are alike in terms of structure, but they have one fundamental difference in functioning mechanism.

Table: Similarities & the Difference between Camera and Human Eye

	Function	Camera	Human Eye
Similarities
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
Difference
1.	focusing mechanism	change distance between lens & film	change focal length of lens using ciliary muscles