How is the earthworm limited by being a skin breather?

Earthworms need oxygen just like humans, but they don't have lungs like we do. They have a special skin that allows them to “breathe” oxygen right through it. ... This means the amount of oxygen inside the earthworm will always be less (lower concentration) than the area outside of the earthworm (higher concentration).

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.