Question

Why do the mammals lack mucus glands in their skin?
A. It is not slippery.
B. It is tough.
C. Skin epidermis is many-layered.
D. It is not respiratory.

Answer 1

Mucus glands produce mucus, which is a slimy slippery substance. This nature of mucus makes it a protective layer. Mammals have mucus glands inside their body, in the linings of various vital organ systems.

Complete answer: The sole purpose of mucus glands is to provide protection to the delicate and moist skin from bacteria. They are present in organisms that respire through their skin, i.e., cutaneous respiration. For cutaneous respiration, the skin is delicate. So, it needs protection. This protection is provided by the mucus glands via the secretion of mucus. In humans, cutaneous respiration is absent. Mammal respire through lungs, i.e., pulmonary respiration. For pulmonary respiration, there is no such requirement for the skin as in the case of cutaneous respiration. That is the reason why mammal skin lacks mucus glands.
So, the answer is option D. Mammal skin is not respiratory in function, hence, no mucus glands are required.

Additional information:
Gaseous exchange in cutaneous respiration is controlled by three factors –
Ventilation – It is the rate of delivery of respiratory medium (water or air) to the respiratory surface.
Diffusion – It is the passage of gases through the skin.
Convection – It is the carrying of dissolved gases towards or away from the lungs.
Cutaneous respiration is seen in a variety of marine, intertidal and freshwater fish. The skin of amphibians is the major site of respiration in all species. It is the sole respiratory mode of lungless salamanders which lack lungs entirely. In frogs and other amphibians, cutaneous respiration may be the primary mode of respiration in colder temperatures.

Note: Skin of animals which respire through the skin is very delicate and moist, so it requires protection from pathogens. This is why their skin has mucus glands. Mammalian skin is not delicate, so it lacks mucus glands.