Explain why :

(a) Skin functions as an accessory excretory organ.

The skin has sweat and sebaceous glands which excrere or eliminate certain substances along with their secretion. The sweat secreted by the  sweat glands contains salts, lactic acid, amino acid etc. Sebaceous glands secrete sebum to keep skin waterproof and hair greasy, but it contains some excretory products like waxes, sterols, fatty acids and some hydrocarbons.