Horseshoe Crabs: Merostomata

Adults horseshoe crabs range in length from 3.5 to 33.5 inches (89 to 850 millimeters). The females are larger than the males. Horseshoe crabs have a large, arched forebody covered by a horseshoe-shaped carapace (KARE-a-pays), or upper shell, followed by a smooth abdomen with spines on the sides, and a thin tail. There are two pairs of simple eyes, or eyes with one lens, on top of the carapace and a pair of compound eyes, or eyes with multiple lenses, on ridges toward the sides. The mouthparts are made up of a pair of pincherlike mouthparts and a pair of clawed leglike appendages. There are four pairs of clawed walking legs. The walking legs have seven segments, the last two form pinchers on the first four pairs of legs. The bases of the fourth pair of legs are fitted with special structures called flabella. The flabella are used to clean the book gills. The last pair of legs ends in four leaflike structures. These legs are used to push through, and sweep away mud, silt, and sand as the horseshoe crab burrows through the sea bottom in search of food.

The solid midsection, or abdomen, has six pairs of flaplike limbs. The first pair is joined together and protects the reproductive opening, through which the crab lays its eggs. The other five pairs form the gills, the organs through which the crab breathes underwater. They are called book gills because they resemble the pages of a book. Movable spines stick out on each side of the midsection. Different species, or types, of horseshoe crabs have different numbers of spines. A long thin tail extends from the end of the midsection and is used for steering through the water. Individual crabs that are trapped on their backs on the beach use the long tail to flip themselves over.