Pupils
By around 550 million years ago, some kinds of worms began to evolve eyespots sunken into their heads to protect them from being damaged. As a side effect, it turned out that being sunken into the head also helped to focus the light on just one spot, so that more light reached that spot and the worm could see better. This also let the worms tell what direction the light was going from. Seeing better gave these worms an advantage, so more of them survived.
But roundworms still could only tell light from darkness, not see a picture the way you do (if you can see). About 500 million years ago, some worms evolved pupils that were sunk so deep that they acted like a pinhole camera to let the worms see an image.
About 400 million years ago, some fish developed pupils that could get bigger or smaller. That way they could let in more or less light: more light, if it was dark, and less light, if it was bright out.
To see how the pupil works, build a pinhole camera
Eyes
Nervous system and senses
Reproduction
Skeletal system
Muscular system
Circulatory system
Plants
Animals
To find out more about cells, check out these books from Amazon.com or from your library:
