Composition, growth, and formation are three differences between the exoskeletons of mollusks and arthropods.
The first difference mentioned was composition. The exoskeleton of a mollusk is largely composed of calcium, whereas the exoskeleton of an arthropod is largely composed of chitin.
The second difference mentioned was growth. Examples of mollusks are clams, oysters, and snails. The exoskeletons of such organisms are shells that hold and protect the internal organs of the organisms. These shells grow with the mollusks. Examples of arthropods include lobsters, crabs, and insects. These organisms are born with a soft exoskeleton that hardens over time. The exoskeletons do not grow with the arthropods. Instead, arthropods molt as they outgrow their exoskeletons.
Formation was the last difference between the exoskeletons of mollusks and arthropods. The exoskeletons of arthropods are segmented, whereas the exoskeletons of mollusks are not.