The Fine Structure of an Organ Spanning Two Joints in the Walking Leg of the Horseshoe Crab

Each walking leg of a horseshoe crab is innervated by a large and a small nerve. For part of its course through the trochanter and femur, the small nerve of the leg is attached to a connective tissue strand and to a small muscle, both of which have their origins in the coxa and their insertions in the femur. The nerve includes axons with diameters ranging from 0.13 to 35.00 μm. Mitochondria in the larger axons are often centrally grouped. The connective tissue strand lies between two groups of axons, and includes fibers having the banded appearance characteristic of collagen. The muscle consists of fibers with diameters of 20 to 55 μm. sarcomere lengths of 4.7 to 6.9 μm, a thin/thick myofilament ratio of 3 to 6:1, and is generally comparable to other Limulus muscle. This report corrects previous inaccuracies in earlier descriptions of the gross morphology of this structure, and clearly establishes the identity and the morphological relationships between the three tissue types of which the organ is composed.