Our common animal blood is red. But another creature is very special. It is called horseshoe crab, commonly known as horseshoe crab. It is an arthropod with a slightly horseshoe-shaped head and chest, a hexagonal shell in the abdomen and a sword-shaped tail, and lives in shallow water. It is special in that its blood is blue.

The horseshoe crab has four eyes, and there are two small eyes at the front end of the head breastplate, about 0.5 mm, which should be perceived in the front of the body. There are also two hemispherical compound eyes on both sides of the head and breastplate. Each compound eye consists of about 1000 small monocles. Only in this way can a complex eye form a clear image. It is convenient to distinguish between the enemy and the enemy in any direction at any time.

There are six pairs of appendages in the chest of horseshoe crab fish head, namely 1 pair of claws, 1 pair of feet and 4 pairs of chest limbs. There are 6 pairs of appendages in the abdomen, namely 1 pair of left and right commissures and 5 pairs of inner limbs. Strangely, there is a group of leaflike gills on the inside of the five outer limb joints, like a page. Each group of pages has about 180 small pages overlapping. Blood vessels are distributed in the gill of the book, and the direct gas exchange between horseshoe crabs and the outside world depends on blood vessels.

Because of these appendages, horseshoe crabs can swim freely on land and in water. Six pairs of appendages on the head and chest are used to walk on land, and horseshoe crabs can swim in the water by flapping their cheeks up and down. Therefore, horseshoe crabs are also amphibians.

The horseshoe crab is unique in its blue blood. We humans are red blood because there is hemoglobin in the blood. Among the known animals, there are three kinds of protein that can transport oxygen: hemoglobin, earthworm hemoglobin and hemocyanin.

The blood of horseshoe crab contains hemocyanin, which is blue with copper ions. Copper ions can combine and separate with oxygen, so hemocyanin can also transport oxygen, and it is the only copper protein that can reversibly combine with oxygen.

The uniqueness of horseshoe crab is not only its hemocyanin, but also its blood cells are primitive, with only one kind of cells: abnormal cells. This kind of cell can release a clotting protein, which makes the invading bacteria coagulate and die.

At present, the scientific community, especially the medical community, is very interested in horseshoe crab blood, and valuable parts are extracted from horseshoe crab blood for the detection of vaccines, drinks and other drugs and foods.