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).

i am going to guess that it is co2.

the menstrual cycle – higher tier. negative feedback mechanisms affect the production of hormones in the menstrual cycle. oestrogen also stimulates the release of lh (luteinising hormone) from the pituitary gland, which in turn controls the production of progesterone.