Answer:
Horseshoe crabs are chelicerates, which means they're made of two segments, the cephalothorax and the abdomen. The cephalothorax is a fusion of the head (cephalon) and chest (thorax), and the tail is part of the abdomen. Trilobites, on the other hand, are composed of 3 parts, cephalon, thorax, and pygidium. In this body plan, the pygidium is the tail and is a separate segment of the body. Due to these differences, it seems that the two species are not closely related, however, it's quite possible that they share a yet-undiscovered common ancestor.
