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.