There are three divisions to the body of the horseshoe crab: the prosoma , the opisthosoma, and the telson. These are sometimes referred to as the cephalothorax, the abdomen, and the tail. (You can mouse over the "Divisions of the Body" in the illustration for a closer look)
The prosoma contains a sizeable intestinal tract with an esophagus and proventriculus (used to grind food), a nervous system concentrated into a bulbous brain, a tubular heart, excretory glands at the bases of the walking legs and connective tissue and cartilagenous plates.
The opithosoma contains chiefly the musculature for the operation of the book gills and the telson, though the horseshoe's 113 distinct muscle groups (comprising over 750 individual muscles) are not limited to this section of the body.
Of the six "pages" of the book gills, the first has an operculum with genital openings and the other five have small sensory branches and gill leaflets. (See appendages for more).