An exoskeleton is a hardened, rigid outer surface to which muscles can be attached. Contractions of the muscles cause jointed segments of the exoskeleton to move relative to each other. The simplest example of an exoskeleton is the shell of a mollusk. Some marine mollusks, such as clams, have shells composed of protein strengthened by crystals of calcium carbonate (a rock-hard material). These shells can be massive, affording significant protection against predators. Some terrestrial mollusks (and snails) generally lack the hard mineral component and are much lighter.
The most complex exoskeletons are found among the arthropods (see Key Concept 31.1). A type of exoskeleton called a cuticle covers the outer surfaces of the arthropod body including its appendages. It is made up of chitin (a nitrogen-containing polysaccharide) secreted by a layer of cells just below the exoskeleton. Chitin stiffens and hardens the cuticle everywhere except at the joints, where flexibility must be retained. Muscles attached to the inner surfaces of the arthropod cuticle move its parts around the joints (see Figure 31.4). Growth of animals with exoskeletons is accommodated by molting (see Figure 31.3). Soft-shelled crabs, a gourmet delicacy, are crabs caught while they are molting.