There are more species of arthropods than any other group in the world,
but many do not have a hard part that can be preserved. You probably
have eaten crab and had to tear or crack the shell. The material that
many arthropods cover themselves with is not bone, it is a material that
will break down easily.
Arthropods have a problem when they grow. Their shell does not grow
with them. They must get rid of the old skeleton and replace it with new
one (called molting). If you walk on the beach, many times you
see these small"dead" crab shells, but they are only
successive molts as the arthropod gets larger.
Some arthropods do have a hard part but it is more like a home
than part of its living body. A barnacle is a marine organism that lives
along the coast or attaches itself to rocks and even boats! It has a
home composed of shell material, where the shrimp like creature lives.
This home allows the barnacle to close its doors and can survive for a
short time without water. When the animal dies, the house is the only
remaining clue of its life.