Earthworm

Earthworms are a large group of segmented worm species found in soils worldwide, playing a foundational role in many ecosystems by burrowing through and ingesting soil, breaking down organic matter such as dead leaves, and excreting nutrient-rich castings that improve soil structure, aeration and fertility, work that led Charles Darwin to devote one of his final books entirely to their ecological importance. Earthworms breathe through their moist skin rather than lungs, which is why they must remain damp and often surface after heavy rain when their burrows flood with water. Most earthworm species are hermaphrodites, each individual possessing both male and female reproductive organs, though they still typically mate with another individual to exchange sperm. Earthworms are considered a keystone group for healthy soil ecosystems and agriculture worldwide, and while common species remain abundant, some regions face ecological concerns from non-native earthworm species altering native forest soils.