Topographic Characteristics
There are three main regions on the underside of the horseshoe crab: the doublure , the vault, and the axial platform. (Mouse over the "Primary Regions" in the illustration.)
The illustration shows the underside of the horseshoe with the appendages removed. The bases of the appendages attach to the axial platform, beginning at the top with the chelicerae, which the crab uses to place food in its mouth and ending with the book gills, which are five branchial "legs" used for propulsion when swimming and for "breathing".
See appendages for detailed drawings.
Illustration (modified) by Carl Shuster