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Merostomata is a class of marine arthropods which includes horseshoe crabs and eurypterids. It includes only four living species.

Classification

• Class Merostomata
  • Order Eurypterida †
  • Order Xiphosura
    • Family Limulidae
      • Genus Carcinoscorpius
      • Genus Limulus
      • Genus Tachypleus

Order Eurypterida

Eurypterids are extinct giant arthropods, likely the largest arthropods to ever inhabit the planet, growing up to 3 meters in length. They lived 200 to 500 million years ago. Entirely aquatic, they lived during the Cambrian through the Permian periods. The fossils indicate that they had a marine ancestor and eventually invaded brackish and freshwater environments. They were predatory and looked more like scorpions than their most immediate relatives, the horseshoe crabs.

Order Xiphosura

Xiphosura contains the four living species of the class. 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 three smaller simple ones atop the carapace. Beneath the carapace they look quite similar to a large spider. They have six pairs of legs for walking, swimming and moving food into the mouth. Behind their legs, they have six book gills, which exchange respiratory gases and are also 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. They use holes in sand to lay eggs. These holes are 10-25cm in height. The female usually lays up 1,000 eggs in this hole and the male releases his sperm onto them so that they can be fertilised. These larvae take about 16 weeks to hatch, and it takes another 6 months to reach adult stage. They can reach sexual maturity in 3 years or more.

The young stay here, light brown in color at first, and migrate into deeper waters as they get older. They used to be harvested for fertilizer, but their only current commercial value is that they are sometimes chopped up for lobster bait.

The other three species of this subclass are found along Asian coasts from Japan and Korea, down to the Philippines.

Reference

http://www.earthlife.net/chelicerata/merostomata.html