The phrase "hard exoskeleton" refers to a strong outer covering or shell that protects and supports the body of certain animals, like insects and crustaceans. Full definition
During the Cambrian Explosion, a period of rapid evolutionary innovation about 500 million years ago when most major animal groups emerge in the fossil record, arthropods with hard exoskeletons and jointed limbs first started to appear. (sciencedaily.com)
These marine invertebrate animals have hard exoskeletons made of calcium carbonate, and are sessile, meaning permanently fixed in one place. (nhm.ac.uk)
Beetles have a very hard exoskeleton (outer skeleton) and hard forewings, like all insects their bodies are made up of three parts: the head, thorax and abdomen. (science-sparks.com)