Arthropods have segmented bodies and linearly segmented appendages, all enclosed in a hardened exoskeleton containing a nitrogen-rich polysaccharide, chitin. In embryos, the body is usually divided into segments along its length, although this is usually less obvious in adults due to segment loss or fusion. The nervous system consists of a brain situated antero-dorsal to the gut, linked to a ventral double nerve cord with paired segmental ganglia. The blood system is open, with a contractile dorsal vessel with paired segmental openings forming the heart.
In adult arthropods alive today, the body is usually formed of two or three sections - the cephalum (head), thorax (chest) and abdomen. Growth is accomplished by generation of a new, larger exoskeleton internally, followed by shedding the old one; hormones regulate this process. Larval forms often differ considerably from the sexually mature adult, so development often involves metamorphosis.