Earthworms breathe air through their skin, which diffuses oxygen from the surrounding environment. Moisture is vital for the earthworm, as it allows oxygen to be retained while carbon dioxide is expelled. The mucus secreted by the earthworm's skin assists in preventing desiccation, a state of extreme dryness.

Earthworms vary in size by species. Some species measure less than half of an inch in length, while others can grow to nearly 10 feet long. All earthworms are hermaphroditic and reproduce sexually. Earthworms are extremely beneficial to soil quality due to their burrowing and their consumption of dead organic material. The earthworm's importance in ecology also extends to its place in the food chain, where many other animals depend upon it for sustenance.