There is no single accessory organ to the integumentary system. Instead, there are several structures that are considered accessory organs of the integumentary system, which is the skin. These include:

* Hair: Provides insulation, protection, and sensory input.

* Nails: Protect the fingertips and toes.

* Sweat glands: Regulate body temperature and excrete waste products.

* Sebaceous glands: Produce oil to lubricate and waterproof the skin.

* Sensory receptors: Detect touch, pressure, temperature, and pain.

So, while there isn't one singular "accessory organ," these structures collectively contribute to the overall function of the integumentary system.