Despite its locomotor, protective and supporting advantages, the Arthropod external skeleton poses problems for a growing animal. The solution that has evolved is the periodic shedding of the skeleton, a process called molting or ecdysis and is controlled by both seasonal hormones and growth. As the process is initiated, the epidermis secretes certain proenzymes at the base of the skeleton. The epidermis now detaches from the skeleton and secretes a new epicuticle or outer epidermal layer. The previously formed proenzymes are then activated and dissociate the epidermis from the exoskeleton.
Muscles attach to the skeleton by tonofilaments in specialized epiderman cells.
Only at the very last moment are these attachments severed. At this point, the animal is encased in both an old and a new skeleton. The old skeleton now splits along the anterior and lateral margins of the carapace, and the horseshoe crab crawls forward out of the old shell. In contrast, when true crabs and lobsters molt, they crawl out of their old shell backward. The horseshoe crab's new skeleton, which is soft at first, is stretched to accommodate the increased body size.