Composition

Exoskeletons for crabs and lobsters are made of chitin, a semi-rigid protein structure. Some species of crab (the hard-shelled varieties) also have calcium carbonate embedded in the chitin, making it heavier and more rigid.

Muscle Attachment

Exoskeletal systems for crabs and lobsters have several attachment points on the inside for muscle. When you eat a crab's legs or a lobster's tail, you're eating the muscle inside the skeleton. The nodules inside the shell are where the muscle attaches, holding the meat in place.

Moulting

When a crab or lobster grows, it sheds its exoskeleton. Moulting involves some risk for the animal as it spends a few days waiting for the new shell to harden.

Exoskeletons and Sensory Organs

Crabs have tympani (eardrum-like organs for sensing sounds in water), which are made out of a thinner layer of the same material as the exoskeleton. Lobsters have antennae that are made of stalks of exoskeleton material and serve as vibration sensors like the tympani. In addition to the vibration senses, lobster antennae have the smell and taste organs at their ends. In both cases, the exoskeleton serves as more than armor for the body of the crustacean.

Segmentation and Movement

Exoskeletons are usually divided into segments; the segment containing the brain and the internal organs is usually more heavily protected. Segments in the body provide attachment points for the legs, and the legs themselves are segmented so that they can move, much the same way a knight's armor is segmented so that the knee joint can bend.

Source:

North Carolina State University: The Exoskeleton