Horseshoe crabs belong to arthropods. There are only five species in the world, all of which live in the ocean, and one of them is distributed in the coastal areas of Fujian and Guangdong in China. Limulus is about 60 cm long and is the largest species of arthropod. Because of its peculiar appearance, coastal fishermen call it ugly. The horseshoe crab is shaped like a gourd ladle and brown in color. It is divided into three parts: head, chest, abdomen and tail of sword. There is a hard shell on the head, back and chest, which looks like a flat helmet. There are a pair of monocular eyes and a pair of compound eyes on the carapace. The head, chest and abdomen are sunken and have 6 pairs of appendages. The first pair is short, called pliers, and the second pair is grown up, called feet. The other four pairs are located on both sides of the mouth and have many functions such as chewing food, swimming and crawling. The last pair of appendages is also suitable for digging holes in sand. There are five pairs of gills on the ventral surface of horseshoe crab, which are folded into pages like books. These gills, called book gills, are the respiratory organs of horseshoe crab. The anus is located behind the gill of the book. The horseshoe crab's sharp tail is its main defensive weapon.
No matter on the bottom of the sea or on the beach, horseshoe crabs often crawl in pairs. Mature male horseshoe crabs grow into a curved hook at the ends of the second pair of appendages on the head and chest. The male horseshoe crab hooks the side of the abdominal armour of the female horseshoe crab, climbs to the back of the female horseshoe crab, and walks in tandem. Once combined, they will never be apart. Usually, horseshoe crabs live on the sandy seabed, relying on one eye and compound eyes to perceive light, identify objects, and look for various worms and mollusks as food.