Biological class of marine arthropods which includes horseshoe crabs and eurypterids.
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Merestomata

This biological class of marine arthropods has only four living species. There are two subclasses:

Table of contents
1 Eurypterida
2 Xiphosura
3 Reference


These are all 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 fed on a variety of kinds of foods and that looked more like scorpions than their immediate relatives are the horseshoe crabs.


These contain 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 that they can plow through with a dark brown, protective carapace, and a tail spike.

They have two large eyes and four smaller ones atop the carapace. Beneath the carapace they look quite similar to a large spider. The entire body of horseshoe crab is protected by carapace. The carapace is said to bear two compound and two simple eyes: these compounds eyes are reduced. They use gills to exchange respiratory gases. They use sepcialized book gills, gills with 100-200 flat plates, to maximize surface area. They have six pairs of legs that are specialized for walking and reproduction. While they can swim upside down, they usually are found on the ocean floor itself searching for worms and mollusks that are their main food. They fed on small worms, crustaceans and even small fishses.

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 10-25m 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 larvaes 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.