In insects, the anterior portion of the heart, located in the abdomen, is extended into a tube, the aorta, which directs the blood forward as it goes out into the body cavity. Arthropods have a well-developed, mesodermal, ventral, solid nerve cord and well-developed sense organs. The body feature from which the phylum takes its name is the jointed appendages, which include antennae and mouthparts as well as walking legs. Arthropods early ancestors had a number of body segments, each with a pair of jointed appendages and looked similar to a centipede. From there, some of these segments became fused to form a head and some of the appendages became modified to form mouthparts or antennae. Early on, there was an evolutionary split which led to the various modern subphyla and classes. Currently, three living subphyla are recognized, with trilobites representing an extinct fourth subphylum (Carter, 1997).

Horseshoe Crabs belong to the subphylum Chelicerata, and are in the class Xiphosura, xipho meaning sword and ura meaning tail. Also in subphylum Chelicerata is class Arachnida, arachni meaning spider. This class includes scorpions, mites, ticks, daddy-long-legs, and spiders.

Subphylum Crustacea, crusta meaning crust or rind, includes crayfish, lobsters, crabs, pillbugs. They have gills, thus terrestrial pillbugs need to maintain a 100% humidity environment around their gills to be able to "breathe." Crustaceans have the head and thorax combined into one body region, the cephalothorax,...

They have two pairs of antennae, mandible-type mouthparts (of different evolutionary origin than mandibles in insects), and other mouthparts which include two pairs of maxillae and three pairs of maxillipeds, all of which are formed from modified appendages.
Subphylum Atelocerata includes insects, centipedes and millipedes. Members of this subphylum have one pair of antennae, mandibles, and two other pairs of mouthparts, either two pairs of maxillae or one pair of maxillae plus a labium, which are modified appendages.

Members of the fourth subphylum, Trilobita, are now all extinct. Their bodies were divided into three sections, lobes, a head and a body region.

References

Carter, J.S. (1997). Phylum Arhropoda. UC Clermont College Biology Home Page. Retrieved September 14, 2010, from http://biology.clc.uc.edu/courses/bio106/arthrpod.htm

NAI. (1999, February). Horseshoe crabs. National geographic, Vol. 195, Issue 2. Retrieved September 14, 2010, from http://web.ebscohost.com/ehost/detail?vid=7&hid=7&sid=69110e3d-d254-4915-a7c7-bed5e36affaf%40sessionmgr12&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=1440406

NAI. (2009, December 1). Horseshoe crab. Columbia electronic encyclopedia, 6th Edition . Retrieved September 14, 2010, from http://web.ebscohost.com/ehost/detail?vid=4&hid=7&sid=2bdf4e92-2120-48e7-9941-1005ce6b0446%40sessionmgr14&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=39012259