This one-way flow is maintained by valves in the posterior region of the heart. The respiratory system transports oxygen throughout the arthropod's body. This is different from vertebrates in that vertebrates use the circulatory system. The resultant impact is a limitation of the size of the organism, with each cell necessarily within diffusion distance of a respiratory structure in the arthropod. The respiratory system of a typical arthropod is numerous, small, branched, cuticle-lined air ducts called tracheoles with an opening to the outside via spiracles that can be closed to reduce water loss.
Malpighian tubules are projections from the digestive tract, between the midgut and the hindgut.
Water loss is regulated because water and salts are reabsorbed by the hindgut and returned to the circulation. The arrangement of appendages in the largest class, Arachnids, is a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. The class feeds by pumping digestive enzymes into its prey and sucking up the digested material. What remains is a dried husk of the original prey. The ancient arthropods that are ancestors to horseshoe crabs are trilobites.