More on Arthropod Morphology

Arthropods have an exoskeleton of a tough compound called chitin. This outer shell provides the structure against which arthropod muscles pull, reduces water loss, and protects them from environmental dangers. To move inside such a rigid armor, the chitin is laid down in plates, with joints between them. This gives the arthropods their name, meaning "jointed feet".

Some arthropods, like the centipedes, millipedes, and insects, have legs with a single branch (uniramous appendages). The rest historically have legs with two branches (biramous appendages). The outer branch is often a flattened gill, while the inner branch is often used for walking or modified for grasping, chewing, or reproduction. A number of arthropods that now apparently have uniramous appendages, such as spiders and scorpions, are descended from ancestors that had biramous appendages: appendages may be modified in such a way that one branch is lost or concealed.

Arthropods also have a hemocoel, an open body cavity in which blood flows and bathes the tissues and organs. The dorsal tubular heart is perforated by pores (ostia); arthropods generally lack blood vessels.