Question

Why are the gills, the moist skin and lungs richly supplied with blood capillaries

Answer 1

The gills are distributed with blood vessels and absorb oxygen through diffusion. The oxygen combines with hemoglobin of blood and forms oxyhaemoglobin which goes to tissue.
Skin absorbs oxygen dissolved in water through blood capillaries where oxygen combines with hemoglobin. It is carried into different parts of the body by blood and release energy.
so they are richley supplied with blood capilaries to absorve dissolved oxygen