How come trilobite eyeballs become fossils? Are they made out of bone?

Hi Ayse, 
unlike our eyes, the eyes of arthropods are not soft balls of 'jelly', but instead have lenses on the surface.  The lens is formed from the same material as the cuticle of the rest of the animal.  The arthropod eye, and therefore that of the trilobite is made up of many repeating units called 'ommatidia', each of which behaves like an individual lens and photo receptor, so the image that a trilobite would see would be very different from the image that our brain might create with its single lens per eyeball. 
Because the lenses are 'hard' they resist decay and can be fossilised.  The link below shows some really good images of the eyes of living animals, in particular insects.

http://www.d.umn.edu/~olse0176/Evolution/insects.html

The final part of my answer is that they are not bone.  None of the hard parts of an arthropod are bone.  An arthropod makes it skeleton from a substances secreted by a single layer of epidermal cells.  The secreted exoskeleton forms rigid plates, attached to a flexible cuticle.  This cuticle remains transparent over the eyes, but can be coloured by the addition of pigments.
This skeleton cannot grow, so must be shed every so often, to allow the arthropod to grow.  When the arthropod has expanded it simply secretes a new cuticle and plates.  These harden  and the animal gets on with its day.

Last edited by Neil Gostling (6th Feb 2007 17:22:36)

Neil didn't mention that each lens of a trilobite eye is made up of an individual crystal of calcite (the rest of the trilobite exoskeleton is made up of chitin and calcite, as well as calcium phosphate). The amount of lenses per eye varies between each species, from none at all to about 15,000!

Our eyes have flexible lenses to allow us to focus on different objects, do you think that a triblobite would be able to focus the lenses in its eyes?

Sarah

p.s. Did you know that an eye with lots of lenses is called a compound eye and is found in lots of insects including spiders and flies?