Spiders are unique regarding their respiratory system. They are the only animal group that breathe simultaneously with lungs and tracheae. Looking at the physiology of respiration the existence of tracheae plays an important role in spiders with a well-developed tracheal system. Some spiders have book lungs. A book lung has a stack of soft plates called lamellae. Oxygen in the air passing between the lamellae diffuses through the tissue into the blood. Other spiders have tracheae which are breathing tubes held open by rings of chitin.