Merostomata

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

Eurypterida[?]

These are all extinct giant arthropods, likely the largest arthropods to ever inhabit the planet, growing up to 3 meters in length. 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.

Xiphosura[?]

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. 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. In late spring, they migrate into shallower waters to spawn, laying their eggs in the upper intertidal zone. 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.