Final answer:
Crabs, being invertebrates within the Arthropods group, have an exoskeleton made of chitin and sometimes calcium carbonate. This external skeleton offers protection, support, and allows for movement. They must molt to accommodate growth since the exoskeleton cannot expand.
Step-by-step explanation:
Invertebrates such as crabs possess an exoskeleton, which is an external skeleton that consists of a hard encasement on the surface of the organism. Specifically, crabs are part of the Arthropods group, whose exoskeleton is made of chitin, a polysaccharide derivative of glucose, and is often further strengthened by calcium carbonate. This exoskeleton provides defence against predators, structural support for the body, and enables movement through the contraction of attached muscles. As crabs grow, they must periodically shed their exoskeleton in a process known as molting, because the exoskeleton does not expand with their growth.