The body of earthworms is covered by a layer of mucus which is secreted by their skin all along their body. This mucus serves several purposes:

  • It lubricates the earthworm’s body so that it can slip into the soil
  • It adheres to the walls of the tunnel opened by the earthworm, leaving ventilation paths in the soil
  • It helps maintain the moisture of the earthworm.

The last point is the most important as earthworms are 80% water when fully hydrated, but water is difficult to maintain. The loss of water is caused by the skin through special resources from which the urine of earthworms comes out. Earthworms must live in an environment with enough moisture to replenish water through their skin and food.
On the opposite side, a flooded environment is not desirable because earthworms breathe through their skin and such an environment does not allow adequate oxygen supply.