Structure C is labeled as a coiled tubular structure extending from the dermis and opening to the skin surface.
Step 2: Recall the Function of Skin Structures
One of the skin's functions is thermoregulation, controlled by eccrine sweat glands. These glands are coiled tubular structures residing in the dermis and secrete sweat through ducts onto the epidermis.
Step 3: Match the Structure with its Function
Structure C matches the description and function of an eccrine sweat gland.
Final Answer: The structure labeled C is an eccrine sweat gland.