蚯蚓是对环节动物门寡毛纲类动物的通称。在科学分类中，它们属于单向蚓目。身体两侧对称，具有分节现象；没有骨骼，在体表覆盖一层具有色素的薄角质层。蛋白质含量达70%，还有微量元素，如磷、钙、铁、钾、锌、铜以及多种维生素。除了身体前两节之外，其余各节均具有刚毛。雌雄同体，异体受精，生殖时借由环带产生卵茧，繁殖下一代。目前已知蚯蚓有200多种，1837年被生物学家达尔文称之为地球上最有价值的动物。

循环系统是封闭式循环系统，消化管为一由前至后延伸的管状构造，排泄则经由肛门或肾管进行，喜食腐质的有机废弃物。以皮肤呼吸，会从背孔分泌黏液以保持皮肤的湿润。古人误以为蚯蚓出土时会发出声音。

An earthworm is a tube-shaped, segmented worm found in the phylum Annelida. Earthworms are commonly found living in soil, feeding on live and dead organic matter. An earthworm's digestive system runs through the length of its body. It conducts respiration through its skin. It has a double transport system composed of coelomic fluid that moves within the fluid-filled coelom and a simple, closed blood circulatory system. It has a central and a peripheral nervous system. The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve cord running back along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut, and their actions move the digesting food toward the worm's anus.