MUSCLE ATTACHMENTS IN HORSESHOE CRAB WALKING LEGS

¹ Department of Biology, Clark University, Worcester, Massachusetts 01610

At their insertion, propodite flexor muscle fibers in horseshoe crabs are attached to apodemes via specialized epidermal cells, the tendinal cells, which contain tremendous numbers of microtubules. At the desmosomal muscle-tendinal cell junction, thin myofilaments attach to one side of the junction and tendinal cell microtubules to the other side. At the apical end of the tendinal cells, the microtubules extend into the dense material lining the hemidesmosomes formed by the tendinal cells and apodeme. In this manner, thin myofilaments are functionally connected to apodemes through intervening tendinal cell microtubules.

At the muscle origin, collagen fibrils embedded in a matrix connect terminal Z lines, which are two to three times the width of normal Z lines, to the surface cuticle. Tendinal cells and microtubules are not involved, and the attachment of muscle is directly to the exoskeleton.