An arthropod(from Greekarthro-, joint +podos, foot) is an invertebrate animalhaving an exoskeleton(external skeleton), a segmentedbody, and jointed appendages. Arthropods form the phylumArthropoda, and include the insects, arachnids,myriapods , and crus .... Arthropods' primary internal cavity is a hemocoel, which accommodates their internal organs, and through which their haemolymph- analogue of blood- circulates; they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous systemis "ladder-like", with paired ventral nerve cordsrunning through all segments and forming paired gangliain each segment. Their heads are formed by fusion of varying numbers of segments, and their brainsare formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratoryand excretorysystems of arthropods vary, depending as much on their environment as on the subphylumto which they belong.