The book lung in the wolf spider, Pardosa astrigera was consisted of a series of flattened triangular-shaped air sacs, stacked with about 70 sheets, and was located in the ventrolateral region of opisthosoma. Each hemolymph spaces (average $8{mu}m$ in thickness) surrounded by the air sacs (average $6{mu}m$ in thickness). The air sacs was supported by cylindrical cuticular spikes of microfibril bundles. Epithelial cell processes surrounded the hemolymph spaces. The nuclei of the epithelial cells were concentrated near the atrium. In the middle portion of air sac, the epithelial cells formed pillars across the hemolymph spaces and spot desmosome and zonula adherens were seen between the plasma membranes. In the hemolymph space of this spider, granular hemocytes (average diameter $8{mu}m$) were the most dominant type of hemocytes. In the medial sinus, the hemolymph flow between the air sacs of a paired book lungs and then flow out of the lung vein. The air comes in the atrium through the ventral lung slit and makes a tidal wave in and out of the air sacs.