The Arthropoda is a phylum consists of invertebrate animals with an external skeleton and a segmented body. Their respiratory system depends on their environment and the subphylum they belong to. Arthropods have tracheae and book lungs as respiratory organs. They have tiny tubes that permit passage of gases into the interior of the body.
Chelicerata
These depend on individual sub-groups' environments. Modern terrestrial chelicerates generally have both book lungs, which deliver oxygen and remove waste gases via the blood, and tracheae, which do the same without using the blood as a transport system. The living horseshoe crabs are aquatic and have book gills that lie in a horizontal plane. However a fossil of the 45 millimetres (1.8 in) long eurypterid Onychopterella, from the Late Ordovician period, has what appear to be three pairs of vertically oriented book gills whose internal structure is very similar to that of scorpions' book lungs.
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains over 13,000 species, all of which are terrestrial. Myriapods breathe through spiracles that connect to a tracheal system similar to that of insects. There is a long tubular heart that extends through much of the body, but usually few, if any, blood vessels.
Hexapoda
Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs.
The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.
The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.