Lungs - Respiratory Organs
In arachnid arthropods such as scorpion and spider, respiration takes place by means of book lungs. There are four pairs of these structures in the scorpion, opening at the four pairs of slit like stigmata on the ventral sternum. Each book lung takes the type of a small hollow sac (pulmonary sac) filled along with clusters of lamellae, 130 to 150 in number. These lamellae are disposed like the leaves of a book. The leaves are borne on an axis. Each leaf is hollow and the blood to be oxygenated flows in the narrow slit such as cavity, separated from the air by membranous walls. The spiracle opens into an atrial chamber. Each lung is enclosed in a pulmonary cavity. The lamellae consist of respiratory air derived from the atrium and blood circulates in the spaces among the lamellae. It is here that exchange of gases occurs through the thin walls of the lamellae. Air enters the lamellae by diffusion, but in some species the atrium is reported to generate a ventilating action by means of muscles.