THe Limulidae (Horseshoe crab)
Horseshoe Crabs generally live in areas with shallow ocean water and sandy or muddy bottoms. Its entire body is covered in a hard shell that protects it. The tail on its back is used to flip itself over when it is on its back; A Horseshoe Crab without its tail is more susceptible to predators. Despite this, Horseshoe crabs actually swim upside down, using their book gills to help propel them. Book gills are used for respiratory gasps and is occasionally used for swimming. Their mouths are located at the center of their legs, which is why they travel so close to the sea floor, looking for small worms and mollusks and occasionally, smaller crustaceans.
The system
The Horseshoe Crab's circulatory system is basically a large tube that branches off through the rest of the body. The heart is a long tube that spans almost the entire vertical length of its body, and has a rate of 32 beats per minute. Blood enters the heart through eight slits called Ostia, and is then pumped forward and out through three types of tubes: three pairs of Aortae, one pair of cerebral atreries, and a frontal artery.