A cuticle is a hard, protective outer layer found on the external body surfaces of various organisms across multiple biological kingdoms. Functionally, it acts as a primary barrier, offering defense against desiccation, pathogens, physical trauma, and chemical irritants. In many arthropods, the cuticle forms part of the exoskeleton and is composed primarily of chitin and proteins, providing structural support. In other taxa, such as vertebrates, the term refers to a keratinized layer found on appendages or epidermal surfaces. The precise composition and structure of the cuticle vary widely; some are highly mineralized, while others are formed of complex lipid and protein matrices designed to regulate water loss and maintain physiological stability.