Due to their respiratory system's limited ability to deliver by diffusion, respiratory gases must be transported around the body."Paralithodes camtschaticus" has an open circulatory system with a dorsal, ostiate heart. An open circulatory system has circulating fluid that somewhat passes freely among the tissues before being collected and recirculated. The heart is in a pericardial chamber, and blood passes through this chamber into the lumen of the heart through two pairs of ostia. Seven arteries conduct blood from the heart to various regions of the body. Each artery branches extensively, and smaller arteries ultimately end in the hemocoel. Venous blood drains into the sternal sinus, where it is conveyed by channels to the gills for aeration and returned again to the pericardial sinus.