An exoskeleton is a type of skeleton that is an external anatomical feature that supports an protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Whilst many, many other invertebrate animals (such as shelled mollusks) have exoskeletons in the sense of external hard parts, the characteristic is most associated with the arthropods (i.e. insects, spiders, myriapods and crustaceans). Exoskeletons contain rigid and resistant components that fulfil a set of functional roles including protection, excretion, sensing, support, feeding and (for terrestrial organisms) acting as a barrier against desiccation. Exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution has been seen as a critical driving role in the Cambrian explosion of animals that took place subsequent to this time.