The answer would be:
Arthropods
Here's more about Arthropods:
Arthropods basically share a simple body plan, even if they make up the largest phylum of kingdom animalia. Their known characteristics are jointed appendages and exoskeleton.
The exoskeleton of arthropods are mostly made up of chitin and they are stiff. This external skeleton provides arthropods with protection, but they do not grow. So periodically, they shed or molt, so it can grow. After it molts, it secretes a new exoskeleton and it eventually hardens.