Members of the phylum Arthropoda are distinguished by having segmented bodies and segmented appendages. The bodies of most arthropods are made of two or three distinct parts-a cephalum (head), a thorax (chest), and an abdomen. They have a hardened external skeleton, often called the exoskeleton, made of the nitrogen-rich polysaccharide chitin. As the animal outgrows the exoskeleton, it is shed and a new one is regenerated. Many arthropods metamorphose: the egg develops into a larval form that differs considerably from the sexually mature adult. Although arthropods range in size from 100 µm to 60 cm long, most adults are about 1 mm long.