What structure provides external protection and support to certain organisms?

The exoskeleton is a vital structural feature for various organisms, especially invertebrates such as arthropods (e.g., insects, crustaceans). It functions as an external protective layer made primarily of chitin, a tough and flexible material. The exoskeleton provides not only physical protection against environmental hazards and predators but also plays a crucial role in maintaining the organism's shape and structure.

Additionally, the exoskeleton contributes to movement through the attachment of muscles that operate against it, allowing for locomotion. Because it is external, it differs fundamentally from an endoskeleton, which is an internal framework found in vertebrates, and it offers distinct advantages in terms of support and protection in the specific environments where these organisms thrive.

The other structures listed, such as armor and shell, can also provide protection but do not fulfill the same role or definition as specifically as the exoskeleton does in this context. Armor is a more general term that can refer to protective outer layers in various organisms, while shells are specifically hard coverings produced by certain animals, such as mollusks, which can be considered a form of exoskeleton but are not as broadly applicable as the term "exoskeleton." Thus, the exoskeleton is