why only moist skin can be a respiratory organ not the dry skin for the animals?
Water is one of the most important requirement in the process of respiration. it is required for the gases to diffuse across the cell membranes. As the moist skin of animals provides water for the respiration to take place, it can function as a respiratory organ in animals while dry skin cannot function like that due to absence of water.
The respiratory system must meet two important criteria. First, the respiratory surface must be large enough to take in oxygen in sufficient quantities to meet the organism's needs and release all waste gas quickly. Some animals, such as the earthworm, use the entire body surface as a respiratoryorgan. The internal respiratory organs of vertebrates generally have many lobes to enlarge the surface area. Second, the respiratory membrane must be moist, since gases require water to diffuse across membranes. The watery environment keeps the respiratory surface moist for aquatic animals. A problem exists for land animals, whose respiratory surfaces can dry out in open air. As a result, animals such as the earthworm must live in damp places. Internal respiratory organs provide an environment that is easier to keep moist.