Figure 3 in Respiration by jumping spiders (Araneae: Salticidae)

Figure 3. Transverse sections through segment VIII (opisthosomal segment II) of spiders, showing the direction of hemolymph flow from medial (medioventral) sinuses through the air pockets of each book lung to the lateral (pulmopericardial) sinuses leading to the pericardium and heart. 1, Diagram based on a section of Pholcus drawn by Willem (1918). 2, Schematic diagram, colored to indicate the oxygenation of hemolymph as it passes through the book lungs. The color here is only schematic, as spider hemolymph, including circulating hemocyanins and hemocytes, is essentially transparent. Simple one-way valves (ostia) only permit flow in one direction, from the pericardium into the heart, during diastole. When the heart contracts (systole) they close. In the pericardial cavity, the heart is suspended by many ligaments.