The exoskeleton of the crabs and spires are very tough. They protect their internal organs.
The exoskeleton is shed periodically during growth and replaced by a
larger one for the growing body.This process is also called moulting or ecdysis. The exoskeleton remains vernerable untill the new shell hardens.This is how, the exoskeleton plays an important role in the growth of crabs, spiders and other arthopods.

hope this helps..