This is one way you can tell them apart. Once a Limulus sheds its old shell, it has a new, soft one that hardens in about 12 hours. Turn one over and take a look at these structures which do much the same things as your lungs. The total surface area is large enough to permit the exchange of oxygen and carbon dioxide. In addition to permitting exchange of these gases, the gill books can sometimes be used by the animal as paddles while swimming upside down in the water.
Human blood is red because it has a red pigment called hemoglobin which contains iron.
The Limulus blood contains copper rather than hemoglobin thus giving the blood its blue color. The animal's blood also contains fantastically sensitive chemicals used by researchers in discovering harmful bacteria called endotoxins, sometimes found in human blood. In short, the blood of this ancient animal might well save your life some day. Limulus has one pair of feeding legs, four pairs of walking-feeding legs, and one longer pair of walking legs that shove the body forward. Its numbers are being greatly reduced due to extensive use as eel bait, for blood research, and by fearful people who throw the animal up high on the beach, leaving it to die in the broiling sun.