Figure 32.4: Blood Flow in the Open Circulatory System of a Crayfish or Lobster Crayfish and lobsters have many arteries, which extend forward and backward from the heart. The heart pumps blood into these arteries, which transport the blood to most parts of the body. The arteries end, however, and then the blood is released from vessels into sinuses and lacunae that are lined with ordinary systemic tissue cells rather than with vascular endothelium. After release from the arteries, blood from the anterior parts of the body flows posteriorly, and blood from the posterior parts of the body flows anteriorly. These flows converge on a ventral sinus (cavity) that runs along the bases of the principal legs. From there the blood returns to the heart. Although not shown here, the blood passes through the gills (which are attached at the bases of the principal legs) in this final step.