Overview of Primary Nerves

How does the horseshoe sense the world around it? Like us, it uses a system of specialized nerves that extend from the brain throughout the body. In the horseshoe crab, there are several large nerves, that supply the crab with information.

From the anterior portion of the brain, a pair of optic nerves runs directly to the two lateral eyes. There are also 8 pairs of haemal nerves that extend into the body of the crab. These contain motor and sensory fibers and are distributed mainly to the membrane and other tissues. The sixth pair sends branches to the heart and intestine.

All of the haemal nerves are essentially the same, except the first one, the lateral line nerve. It runs close to the surface, just outside the bases of the appendages, begins to branch at the base of the 6th leg. It extends the whole length of the branchial chamber, sending one small branch to the base of each of the five gills. It is a purely sensory nerve and supplies the skin lining the channel along which water is carried to the gills.

Top illustration (modified) by William Patten