This is a picture of a molted horseshoe crab shell. It represents an exoskeleton. An exoskeleton is a rigid external skeleton that protects and supports the animal within it. The exoskeleton has advantages and disadvantages: it is much stronger than an endoskeleton, but it cannot increase in size, so the animal has to molt to grow (often leaving behind a shell like the one pictured). Many insects and ocean invertebrates have an exoskeleton, the horseshoe crab being one of them.