Limulidae
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 Pair of horseshoe crabs
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Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species known of the class Xiphosura, formerly called Merostomata. The best known species is the horseshoe crab (Limulus polyphemus), whose ancestors can first be seen in the Devonian period's fossil record. They are found along the northwestern Atlantic coast and in the Gulf of Mexico. Horseshoe crabs are found in shallow water on soft sandy bottoms.
The entire body of horseshoe crab is protected by a hard, dark brown carapace. They have two large compound eyes and multiple smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have five pairs of legs for walking, swimming and moving food into the mouth. Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming. While they can swim upside down, they usually are found on the ocean floor searching for worms and mollusks, which are their main food. They may also feed on crustaceans and even small fish.
In the spring, Horseshoe crabs migrate to certain shallow coastal waters. Males select a female and cling onto her back. The female digs a hole in the sand and lays her eggs while the male fertilizes them. The female can lay between 60,000-120,000 eggs in batches of a few thousand at a time. The eggs take about 2 weeks to hatch. The larvae continue to molt six times during the first year. It takes 11 years to reach sexual maturity, after which they may live up to 14 more years.
Horseshoe crabs were formerly harvested for use as fertilizer. Currently they are harvested for their blood, which contains a chemical called Limulus Amoebocyte Lysate (LAL) that is used to detect pathogens and their endotoxins, and used as bait for the eel and conch fisheries.
The other three species of this subclass are found along Asian coasts from Japan and Korea, down to the Philippines and in India.
