The fifth set of appendages are well suited to pushing the horseshoe crab across the sandy bottom. The coxa of these legs has a small exopodite (external branch of the appendages) called the flabellum (highlighted in yellow).
There are approximately one million sensory cells in this organ alone, which the limulus uses to test the composition of the water. For more about these and other neural receptors, see the Nervous System.
Illustrations of appendage details by Jeffrey Shultz