Horseshoe Crabs

The horseshoe crab's body is composed of two parts: the cephalothorax and the abdomen. The cephalothorax is basically the crab's head and thorax fused together. Under the cephalothorax, there are six body segments, each equipped with a pair of limbs. Under the abdominal shell is located the circulatory, respiratory, reproductive, and nervous systems. Further, the abdomen houses part of the crab's digestive system and an abundant number of glands.

Like all members of the subphylum Chelicerata—but unlike other anthropods—the horseshoe crab does not have antennae. Instead, it uses its first pair of appendages (called cheliceras), located in front and to the sides of its mouth, to feed itself. The cheliceras, and all of their appendages except for their walking legs, are equipped with pinchers (called chelas) with which the animal grabs food from the sea floor. The second pair of legs (called the pedipalp) evidently used to be used for walking, but, over time, evolved more specialized functions. Currently, the second pair of legs are used in different ways, depending on the species; basically, these legs can be used for gripping, chewing, or sensing.

While the horseshoe crab does not have a conventional jaw, its four pairs of walking legs have special equipment attached to them. Known as gnathobases, these are primitive devices that the crab uses to manipulate A horseshoe crab moving along the waters edge. Horseshoe crab, photograph. © John M. Burnley/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. and shred food before passing it to its mouth. The last pair of walking legs can be used to break shells and to crush tough food. Because the crab often swallows sand and shell fragments, its gizzard is quite powerful and can grind up almost anything it consumes.

The horseshoe crab has two sets of eyes. The first pair are large and compound, meaning that they are composed of numerous simple eyes clustered tightly together. These large eyes are located far apart on the front side of the dorsal plate. Much less noticeable, the two small, simple eyes are located fairly close to each other at the anterior of the crab's back. Little is known about the animal's other senses.