Our common animal blood is red. But another kind of 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. What makes it special is that its blood is blue.

Limulus 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 brightness at the front of the body. There are also two hemispherical compound eyes on both sides of the head and breastplate, and each compound eye is composed of a small monocle of about 1000. Such a complex eye can form a clear image. It is convenient to distinguish between friends and enemies and hazards in any direction at any time.

There are six pairs of appendages in the chest of horseshoe crab, namely 1 pair of pincers, 1 pair of feet and 4 pairs of chest limbs. There are also six pairs of appendages in the abdomen, namely 1 pair of left and right commissures, and 5 pairs of inner limbs. Strangely, there are a group of leaflike gills on the inner side of the five outer limb joints, which are like pages, and each group of pages has about 180 sheets overlapping. Blood vessels are distributed in the gills of the book, and the direct gas exchange between the horseshoe crab and the outside world depends on blood vessels.

Because of these appendages, horseshoe crab can swim freely on land and in water. Six pairs of appendages on the head and chest are used for walking on land, and the horseshoe crab can swim in the water by flapping its cheeks up and down. Therefore, horseshoe crab is also an amphibious animal.

The unique feature of horseshoe crab is its blue blood. We humans are red blood because there is hemoglobin in the blood. Among that known animal, 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 be combined with and separated from oxygen, so hemocyanin can also transport oxygen, and it is the only copper protein that can be reversibly combined with oxygen.

The unique characteristics of horseshoe crab are not only its hemocyanin, but also its blood cells are primitive, with only one kind of cells: deformed cells. This kind of cell can release a coagulation 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.