The horseshoe crab has an open circulatory system and has blood that is blue, due to the hemocyanin in the blood. The heart of the crab is a long tube that extends almost the entire length of its body. The average heartbeat of the crab is 32 beats per minute. These crabs also have eight pairs of openings found along the heart, in which blood enters the heart from the pericardial chamber. Horseshoe crabs also have gills, which are used for gas exchange.