It may come as a surprise to learn that the familiar horseshoe crab is not a crab at all but rather a member of a huge group of animals that includes spiders, mites, and ticks. Certain mites and bizarre creatures known as sea spiders are, along with the horseshoe crab, the only marine members of this phylum.

The horseshoe crab and related creatures possess a chitinous exoskeleton that is shed as the animal grows. Members of this phylum have a body that is divided into two distinct parts: the cephalothorax, which contains the head and the legs; and the abdomen. The jaws, or chelicera, take the form of stabbing or grinding mouth parts and lend the phylum its common name.

The smooth texture and domed shape of the horseshoe crab's exoskeleton allows it to easily burrow through the sand and mud substrates that it favors. The shell itself is quite interesting to behold, as it usually is colonized by a fascinating array of marine algae, tubeworms, barnacles, sea anemones, and other organisms. Horseshoe crabs are quite active creatures, and both in the wild and in the aquarium they appear to be perpetually searching for the worms, soft-shelled clams, and other creatures that make up their diet.

The horseshoe crab's long tail, more properly termed the telson, is believed by many people to function as a weapon of defense. Actually, these creatures are quite harmless, with the telson's only function being to assist the animal in righting itself after a wave has deposited it on its back. Among this odd animal's other unusual characteristics are its book lungs, unique in the animal world and used for swimming as well as respiration. Horseshoe crabs engage in massive breeding migrations. See the individual species accounts in Chapter 13 for a description of this amazing natural phenomenon.