Why organisms does not need lungs or gills to obtain the oxygen?

Some organisms, such as certain amphibians, worms, and single-celled organisms, can directly absorb oxygen through their skin or cell membranes in a process called diffusion. This method is effective in moist environments where oxygen can easily pass through their surfaces. As long as they have a sufficiently large surface area relative to their volume and live in environments where oxygen is adequately dissolved in water or moisture, they can meet their respiratory needs without specialized structures like lungs or gills.